bash: Add completion for gitk --merge

Option is only completed when .git/MERGE_HEAD is present.

Signed-off-by: Richard Quirk <richard.quirk@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1346,9 +1346,14 @@ _git ()
 _gitk ()
 {
 	local cur="${COMP_WORDS[COMP_CWORD]}"
+	local g="$(git rev-parse --git-dir 2>/dev/null)"
+	local merge=""
+	if [ -f $g/MERGE_HEAD ]; then
+		merge="--merge"
+	fi
 	case "$cur" in
 	--*)
-		__gitcomp "--not --all"
+		__gitcomp "--not --all $merge"
 		return
 		;;
 	esac