Set the `TERM` env variable (#3420)

Resolves #3419

I have tested this change with all the pagers shown in [the
docs](https://github.com/jesseduffield/lazygit/blob/master/docs/Custom_Pagers.md).
Are there others that people frequently use and I should test?

A nice side effect of setting `TERM=dumb` is that `less` now correctly
complains (e.g. when forgetting to set `--paging=never` for delta:


![image](https://github.com/jesseduffield/lazygit/assets/4602612/33e9c048-6ab0-4196-95f6-86ee8effc873)


## Pagers Tested

### none

![image](https://github.com/jesseduffield/lazygit/assets/4602612/4e408fe6-5f19-4142-9183-de56fd738962)

### Delta

![image](https://github.com/jesseduffield/lazygit/assets/4602612/90dd12fa-ed58-4d49-9f71-da5e57f63a74)

```yaml
git:
  paging:
    colorArg: always
    pager: delta --paging=never
```

#### diff-so-fancy

![image](https://github.com/jesseduffield/lazygit/assets/4602612/930e6b93-904e-49a2-bfc2-9b5d9639f514)

```yaml
git:
  paging:
    colorArg: always
    pager: diff-so-fancy
```

#### ydiff

![image](https://github.com/jesseduffield/lazygit/assets/4602612/c2464f0c-f34f-4ebc-9624-8bc0350d92a7)

```yaml
git:
  paging:
    colorArg: always
    pager: ydiff -p cat
```

#### difft

![image](https://github.com/jesseduffield/lazygit/assets/4602612/599dee7a-6568-40e4-9213-76ba643d651f)

```yaml
git:
  paging:
    externalDiffCommand: difft --color=always
```

1 files changed, 25 insertions, 0 deletions

diff --git a/pkg/gui/pty.go b/pkg/gui/pty.go
index 3f59373b7..cf3176f75 100644
--- a/pkg/gui/pty.go
+++ b/pkg/gui/pty.go
@@ -12,6 +12,7 @@ import (
 	"github.com/creack/pty"
 	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/utils"
+	"github.com/samber/lo"
 )
 
 func (gui *Gui) desiredPtySize() *pty.Winsize {
@@ -54,6 +55,13 @@ func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 
 	cmdStr := strings.Join(cmd.Args, " ")
 
+	// This communicates to pagers that we're in a very simple
+	// terminal that they should not expect to have much capabilities.
+	// Moving the cursor, clearing the screen, or querying for colors are among such "advanced" capabilities.
+	// Context: https://github.com/jesseduffield/lazygit/issues/3419
+	cmd.Env = removeExistingTermEnvVars(cmd.Env)
+	cmd.Env = append(cmd.Env, "TERM=dumb")
+
 	cmd.Env = append(cmd.Env, "GIT_PAGER="+pager)
 
 	manager := gui.getManager(view)
@@ -87,3 +95,20 @@ func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 
 	return nil
 }
+
+func removeExistingTermEnvVars(env []string) []string {
+	return lo.Filter(env, func(envVar string, _ int) bool {
+		return !isTermEnvVar(envVar)
+	})
+}
+
+// Terminals set a variety of different environment variables
+// to identify themselves to processes. This list should catch the most common among them.
+func isTermEnvVar(envVar string) bool {
+	return strings.HasPrefix(envVar, "TERM=") ||
+		strings.HasPrefix(envVar, "TERM_PROGRAM=") ||
+		strings.HasPrefix(envVar, "TERM_PROGRAM_VERSION=") ||
+		strings.HasPrefix(envVar, "TERMINAL_EMULATOR=") ||
+		strings.HasPrefix(envVar, "TERMINAL_NAME=") ||
+		strings.HasPrefix(envVar, "TERMINAL_VERSION_")
+}