Improve preview window update events

- Update preview window even if there is no match for the query string
  if any of the placeholder expressions evaluates to a non-empty string.
- Also, if the command template contains {q}, preview window will be
  updated if the query string changes even though the focus remains on
  the same item.

An example:

    git log --oneline --color=always |
       fzf --reverse --ansi --preview \
       '[ -n {1} ] && git show --color=always {1} || git show --color=always {q}'

Close #1307

1 files changed, 29 insertions, 0 deletions

diff --git a/test/test_go.rb b/test/test_go.rb
index 3c487b4b..4f4f94e1 100644
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -1355,6 +1355,35 @@ class TestGoFZF < TestBase
     tmux.until { |_| %w[1 2 3] == File.readlines(tempname).map(&:chomp) }
   end
 
+  def test_preview_flags
+    tmux.send_keys %(seq 10 | sed 's/^/:: /; s/$/  /' |
+        #{FZF} --multi --preview 'echo {{2}/{s2}/{+2}/{+s2}/{q}}'), :Enter
+    tmux.until { |lines| lines[1].include?('{1/1  /1/1  /}') }
+    tmux.send_keys '123'
+    tmux.until { |lines| lines[1].include?('{////123}') }
+    tmux.send_keys 'C-u', '1'
+    tmux.until { |lines| lines.match_count == 2 }
+    tmux.until { |lines| lines[1].include?('{1/1  /1/1  /1}') }
+    tmux.send_keys :BTab
+    tmux.until { |lines| lines[1].include?('{10/10  /1/1  /1}') }
+    tmux.send_keys :BTab
+    tmux.until { |lines| lines[1].include?('{10/10  /1 10/1   10  /1}') }
+    tmux.send_keys '2'
+    tmux.until { |lines| lines[1].include?('{//1 10/1   10  /12}') }
+    tmux.send_keys '3'
+    tmux.until { |lines| lines[1].include?('{//1 10/1   10  /123}') }
+  end
+
+  def test_preview_q_no_match
+    tmux.send_keys %(: | #{FZF} --preview 'echo foo {q}'), :Enter
+    tmux.until { |lines| lines.match_count == 0 }
+    tmux.until { |lines| !lines[1].include?('foo') }
+    tmux.send_keys 'bar'
+    tmux.until { |lines| lines[1].include?('foo bar') }
+    tmux.send_keys 'C-u'
+    tmux.until { |lines| !lines[1].include?('foo') }
+  end
+
   def test_no_clear
     tmux.send_keys "seq 10 | fzf --no-clear --inline-info --height 5 > #{tempname}", :Enter
     prompt = '>   < 10/10'