Fix "git-commit -C $tag"

The scripted version might not have handled this correctly
either, but the version rewritten in C definitely does not grok
this and complains $tag is not a commit object.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -601,7 +601,7 @@ static int parse_and_validate_options(int argc, const char *argv[],
 
 		if (get_sha1(use_message, sha1))
 			die("could not lookup commit %s", use_message);
-		commit = lookup_commit(sha1);
+		commit = lookup_commit_reference(sha1);
 		if (!commit || parse_commit(commit))
 			die("could not parse commit %s", use_message);
 

t/t7501-commit.sh

@@ -327,4 +327,22 @@ test_expect_success 'amend using the message from another commit' '
 
 '
 
+test_expect_success 'amend using the message from a commit named with tag' '
+
+	git reset --hard &&
+	test_tick &&
+	git commit --allow-empty -m "old commit" &&
+	old=$(git rev-parse --verify HEAD) &&
+	git tag -a -m "tag on old" tagged-old HEAD &&
+	test_tick &&
+	git commit --allow-empty -m "new commit" &&
+	new=$(git rev-parse --verify HEAD) &&
+	test_tick &&
+	git commit --allow-empty --amend -C tagged-old &&
+	git show --pretty="format:%ad %s" "$old" >expected &&
+	git show --pretty="format:%ad %s" HEAD >actual &&
+	diff -u expected actual
+
+'
+
 test_done