allow fast flicking through any list panel

Up till now our approach to rendering things like file diffs, branch logs, and
commit patches, has been to run a command on the command line, wait for it to
complete, take its output as a string, and then write that string to the main
view (or secondary view e.g. when showing both staged and unstaged changes of a
file).

This has caused various issues. For once, if you are flicking through a list of
files and an untracked file is particularly large, not only will this require
lazygit to load that whole file into memory (or more accurately it's equally
large diff), it also will slow down the UI thread while loading that file, and
if the user continued down the list, the original command might eventually
resolve and replace whatever the diff is for the newly selected file.

Following what we've done in lazydocker, I've added a tasks package for when you
need something done but you want it to cancel as soon as something newer comes
up. Given this typically involves running a command to display to a view, I've
added a viewBufferManagerMap struct to the Gui struct which allows you to define
these tasks on a per-view basis.

viewBufferManagers can run files and directly write the output to their view,
meaning we no longer need to use so much memory.

In the tasks package there is a helper method called NewCmdTask which takes a
command, an initial amount of lines to read, and then runs that command, reads
that number of lines, and allows for a readLines channel to tell it to read more
lines. We read more lines when we scroll or resize the window.

There is an adapter for the tasks package in a file called tasks_adapter which
wraps the functions from the tasks package in gui-specific stuff like clearing
the main view before starting the next task that wants to write to the main
view.

I've removed some small features as part of this work, namely the little headers
that were at the top of the main view for some situations. For example, we no
longer show the upstream of a selected branch. I want to re-introduce this in
the future, but I didn't want to make this tasks system too complicated, and in
order to facilitate a header section in the main view we'd need to have a task
that gets the upstream for the current branch, writes it to the header, then
tells another task to write the branch log to the main view, but without
clearing inbetween. So it would get messy. I'm thinking instead of having a
separate 'header' view atop the main view to render that kind of thing (which
can happen in another PR)

I've also simplified the 'git show' to just call 'git show' and not do anything
fancy when it comes to merge commits.

I considered using this tasks approach whenever we write to a view. The only
thing is that the renderString method currently resets the origin of a view and
I don't want to lose that. So I've left some in there that I consider harmless,
but we should probably be just using tasks now for all rendering, even if it's
just strings we can instantly make.

1 files changed, 23 insertions, 45 deletions

diff --git a/pkg/commands/git.go b/pkg/commands/git.go
index f72280d64..fb4ddab79 100644
--- a/pkg/commands/git.go
+++ b/pkg/commands/git.go
@@ -176,8 +176,8 @@ func stashEntryFromLine(line string, index int) *StashEntry {
 }
 
 // GetStashEntryDiff stash diff
-func (c *GitCommand) GetStashEntryDiff(index int) (string, error) {
-	return c.OSCommand.RunCommandWithOutput("git stash show -p --color stash@{%d}", index)
+func (c *GitCommand) ShowStashEntryCmdStr(index int) string {
+	return fmt.Sprintf("git stash show -p --color stash@{%d}", index)
 }
 
 // GetStatusFiles git status files
@@ -538,7 +538,8 @@ func (c *GitCommand) PrepareCommitAmendSubProcess() *exec.Cmd {
 // Currently it limits the result to 100 commits, but when we get async stuff
 // working we can do lazy loading
 func (c *GitCommand) GetBranchGraph(branchName string) (string, error) {
-	return c.OSCommand.RunCommandWithOutput("git log --graph --color --abbrev-commit --decorate --date=relative --pretty=medium -100 %s", branchName)
+	cmdStr := c.GetBranchGraphCmdStr(branchName)
+	return c.OSCommand.RunCommandWithOutput(cmdStr)
 }
 
 func (c *GitCommand) GetUpstreamForBranch(branchName string) (string, error) {
@@ -551,41 +552,12 @@ func (c *GitCommand) Ignore(filename string) error {
 	return c.OSCommand.AppendLineToFile(".gitignore", filename)
 }
 
-// Show shows the diff of a commit
-func (c *GitCommand) Show(sha string) (string, error) {
-	show, err := c.OSCommand.RunCommandWithOutput("git show --color --no-renames %s", sha)
-	if err != nil {
-		return "", err
-	}
-
-	// if this is a merge commit, we need to go a step further and get the diff between the two branches we merged
-	revList, err := c.OSCommand.RunCommandWithOutput("git rev-list -1 --merges %s^...%s", sha, sha)
-	if err != nil {
-		// turns out we get an error here when it's the first commit. We'll just return the original show
-		return show, nil
-	}
-	if len(revList) == 0 {
-		return show, nil
-	}
-
-	// we want to pull out 1a6a69a and 3b51d7c from this:
-	// commit ccc771d8b13d5b0d4635db4463556366470fd4f6
-	// Merge: 1a6a69a 3b51d7c
-	lines := utils.SplitLines(show)
-	if len(lines) < 2 {
-		return show, nil
-	}
-
-	secondLineWords := strings.Split(lines[1], " ")
-	if len(secondLineWords) < 3 {
-		return show, nil
-	}
+func (c *GitCommand) ShowCmdStr(sha string) string {
+	return fmt.Sprintf("git show --color --no-renames %s", sha)
+}
 
-	mergeDiff, err := c.OSCommand.RunCommandWithOutput("git diff --color %s...%s", secondLineWords[1], secondLineWords[2])
-	if err != nil {
-		return "", err
-	}
-	return show + mergeDiff, nil
+func (c *GitCommand) GetBranchGraphCmdStr(branchName string) string {
+	return fmt.Sprintf("git log --graph --color --abbrev-commit --decorate --date=relative --pretty=medium %s", branchName)
 }
 
 // GetRemoteURL returns current repo remote url
@@ -606,6 +578,12 @@ func (c *GitCommand) CheckRemoteBranchExists(branch *Branch) bool {
 
 // Diff returns the diff of a file
 func (c *GitCommand) Diff(file *File, plain bool, cached bool) string {
+	// for now we assume an error means the file was deleted
+	s, _ := c.OSCommand.RunCommandWithOutput(c.DiffCmdStr(file, plain, cached))
+	return s
+}
+
+func (c *GitCommand) DiffCmdStr(file *File, plain bool, cached bool) string {
 	cachedArg := ""
 	trackedArg := "--"
 	colorArg := "--color"
@@ -621,9 +599,7 @@ func (c *GitCommand) Diff(file *File, plain bool, cached bool) string {
 		colorArg = ""
 	}
 
-	// for now we assume an error means the file was deleted
-	s, _ := c.OSCommand.RunCommandWithOutput("git diff %s %s %s %s", colorArg, cachedArg, trackedArg, fileName)
-	return s
+	return fmt.Sprintf("git diff %s %s %s %s", colorArg, cachedArg, trackedArg, fileName)
 }
 
 func (c *GitCommand) ApplyPatch(patch string, flags ...string) error {
@@ -908,11 +884,17 @@ func (c *GitCommand) GetCommitFiles(commitSha string, patchManager *PatchManager
 
 // ShowCommitFile get the diff of specified commit file
 func (c *GitCommand) ShowCommitFile(commitSha, fileName string, plain bool) (string, error) {
+	cmdStr := c.ShowCommitFileCmdStr(commitSha, fileName, plain)
+	return c.OSCommand.RunCommandWithOutput(cmdStr)
+}
+
+func (c *GitCommand) ShowCommitFileCmdStr(commitSha, fileName string, plain bool) string {
 	colorArg := "--color"
 	if plain {
 		colorArg = ""
 	}
-	return c.OSCommand.RunCommandWithOutput("git show --no-renames %s %s -- %s", colorArg, commitSha, fileName)
+
+	return fmt.Sprintf("git show --no-renames %s %s -- %s", colorArg, commitSha, fileName)
 }
 
 // CheckoutFile checks out the file for the given commit
@@ -1098,10 +1080,6 @@ func (c *GitCommand) CreateLightweightTag(tagName string, commitSha string) erro
 	return c.OSCommand.RunCommand("git tag %s %s", tagName, commitSha)
 }
 
-func (c *GitCommand) ShowTag(tagName string) (string, error) {
-	return c.OSCommand.RunCommandWithOutput("git tag -n99 %s", tagName)
-}
-
 func (c *GitCommand) DeleteTag(tagName string) error {
 	return c.OSCommand.RunCommand("git tag -d %s", tagName)
 }