merge: fix GIT_EDITOR override for commit hook

Don't set GIT_EDITOR to ":" when calling prepare-commit-msg hook if the
editor is going to be called (e.g. with "merge -e").

Signed-off-by: Benoit Pierre <benoit.pierre@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -821,7 +821,7 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 	if (0 < option_edit)
 		strbuf_commented_addf(&msg, _(merge_editor_comment), comment_line_char);
 	write_merge_msg(&msg);
-	if (run_commit_hook(1, get_index_file(), "prepare-commit-msg",
+	if (run_commit_hook(0 < option_edit, get_index_file(), "prepare-commit-msg",
 			    git_path("MERGE_MSG"), "merge", NULL))
 		abort_commit(remoteheads, NULL);
 	if (0 < option_edit) {