merge-base: free() allocated "struct commit **" list

Fix a memory leak in 53eda89 (merge-base: teach "git merge-base"
to drive underlying merge_bases_many(), 2008-07-30) by calling free()
on the "struct commit **" list used by "git merge-base".

This gets e.g. "t6010-merge-base.sh" closer to passing under
SANITIZE=leak, it failed 8 tests before when compiled with that
option, and now fails only 5 tests.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-base.c

@@ -138,6 +138,7 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	int rev_nr = 0;
 	int show_all = 0;
 	int cmdmode = 0;
+	int ret;
 
 	struct option options[] = {
 		OPT_BOOL('a', "all", &show_all, N_("output all common ancestors")),
@@ -186,5 +187,7 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	ALLOC_ARRAY(rev, argc);
 	while (argc-- > 0)
 		rev[rev_nr++] = get_commit_reference(*argv++);
-	return show_merge_base(rev, rev_nr, show_all);
+	ret = show_merge_base(rev, rev_nr, show_all);
+	free(rev);
+	return ret;
 }