builtin/: add UNLEAKs

Add some UNLEAKs where we are about to return from `cmd_*`. UNLEAK the
variables in the same order as we've declared them. While addressing
`msg` in builtin/tag.c, convert the existing `strbuf_release()` calls as
well.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -466,5 +466,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 	result = diff_result_code(&rev.diffopt, result);
 	if (1 < rev.diffopt.skip_stat_unmatch)
 		refresh_index_quietly();
+	UNLEAK(rev);
+	UNLEAK(ent);
+	UNLEAK(blob);
 	return result;
 }