zsh completion: Add support for per-command completion triggers.

Allow users to configure custom completion triggers on a per-command
basis. For example, if FZF_COMPLETION_TRIGGER='**' and
FZF_PER_CMD_COMPLETION_TRIGGERS[ssh]='', then:

ssh<space><tab> triggers fzf completion.

but

ls<space><tab> will not.

shell/completion.zsh

@@ -4,10 +4,11 @@
 #  / __/ / /_/ __/
 # /_/   /___/_/ completion.zsh
 #
-# - $FZF_TMUX               (default: 0)
-# - $FZF_TMUX_OPTS          (default: '-d 40%')
-# - $FZF_COMPLETION_TRIGGER (default: '**')
-# - $FZF_COMPLETION_OPTS    (default: empty)
+# - $FZF_TMUX                        (default: 0)
+# - $FZF_TMUX_OPTS                   (default: '-d 40%')
+# - $FZF_COMPLETION_TRIGGER          (default: '**')
+# - $FZF_PER_CMD_COMPLETION_TRIGGERS (default: none)
+# - $FZF_COMPLETION_OPTS             (default: empty)
 
 # Both branches of the following `if` do the same thing -- define
 # __fzf_completion_options such that `eval $__fzf_completion_options` sets
@@ -274,8 +275,11 @@ fzf-completion() {
 
   cmd=$(__fzf_extract_command "$LBUFFER")
 
+  # Use a per-command completion trigger if available. Otherwise, fall
+  # back to $FZF_COMPLETION_TRIGGER.
+  trigger=${FZF_PER_CMD_COMPLETION_TRIGGERS[$cmd]-${FZF_COMPLETION_TRIGGER-'**'}}
+
   # Explicitly allow for empty trigger.
-  trigger=${FZF_COMPLETION_TRIGGER-'**'}
   [ -z "$trigger" -a ${LBUFFER[-1]} = ' ' ] && tokens+=("")
 
   # When the trigger starts with ';', it becomes a separate token

test/test_go.rb

@@ -2648,6 +2648,14 @@ class TestZsh < TestBase
       tmux.send_keys 'C-c'
     end
   end
+
+  def test_per_cmd_completion_triggers
+    tmux.send_keys 'declare -A FZF_PER_CMD_COMPLETION_TRIGGERS', :Enter
+    tmux.send_keys 'FZF_PER_CMD_COMPLETION_TRIGGERS[ls]=""', :Enter
+    tmux.send_keys 'ls /', :Tab
+    tmux.until { |lines| assert_operator lines.match_count, :>, 0 }
+    tmux.send_keys :Enter
+  end
 end
 
 class TestFish < TestBase