tag: --force does not have to warn when creating tags

"git tag --force" mentions what old tag object is being replaced
when it is used to update an existing tag, but it shows the same
message when creating a new one.  Stop doing that, as it does not
add any information.

Add a test for this and also to ensure --force can replace tags at
all.

Signed-off-by: Phil Hord <hordp@cisco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -584,7 +584,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		die(_("%s: cannot lock the ref"), ref.buf);
 	if (write_ref_sha1(lock, object, NULL) < 0)
 		die(_("%s: cannot update the ref"), ref.buf);
-	if (force && hashcmp(prev, object))
+	if (force && !is_null_sha1(prev) && hashcmp(prev, object))
 		printf(_("Updated tag '%s' (was %s)\n"), tag, find_unique_abbrev(prev, DEFAULT_ABBREV));
 
 	strbuf_release(&buf);

t/t7004-tag.sh

@@ -104,6 +104,18 @@ test_expect_success 'creating a tag using HEAD directly should succeed' '
 	tag_exists myhead
 '
 
+test_expect_success '--force can create a tag with the name of one existing' '
+	tag_exists mytag &&
+	git tag --force mytag &&
+	tag_exists mytag'
+
+test_expect_success '--force is moot with a non-existing tag name' '
+	git tag newtag >expect &&
+	git tag --force forcetag >actual &&
+	test_cmp expect actual
+'
+git tag -d newtag forcetag
+
 # deleting tags:
 
 test_expect_success 'trying to delete an unknown tag should fail' '