bash completion: only show 'log --merge' if merging

The gitk completion only shows --merge if MERGE_HEAD is present.
Do it the same way for git-log completion.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1003,6 +1003,11 @@ _git_log ()
 	__git_has_doubledash && return
 
 	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
 	--pretty=*)
 		__gitcomp "$__git_log_pretty_formats
@@ -1030,7 +1035,7 @@ _git_log ()
 			--decorate
 			--walk-reflogs
 			--parents --children
-			--merge
+			$merge
 			$__git_diff_common_options
 			--pickaxe-all --pickaxe-regex
 			"