Add regression tests for stricter tag fsck'ing

The intent of the new test case is to catch general breakages in
the fsck_tag() function, not so much to test it extensively, trying to
strike the proper balance between thoroughness and speed.

While it *would* have been nice to test the code path where fsck_object()
encounters an invalid tag object, this is not possible using git fsck: tag
objects are parsed already before fsck'ing (and the parser already fails
upon such objects).

Even worse: we would not even be able write out invalid tag objects
because git hash-object parses those objects, too, unless we resorted to
really ugly hacks such as using something like this in the unit tests
(essentially depending on Perl *and* Compress::Zlib):

	hash_invalid_object () {
		contents="$(printf '%s %d\0%s' "$1" ${#2} "$2")" &&
		sha1=$(echo "$contents" | test-sha1) &&
		suffix=${sha1#??} &&
		mkdir -p .git/objects/${sha1%$suffix} &&
		echo "$contents" |
		perl -MCompress::Zlib -e 'undef $/; print compress(<>)' \
			> .git/objects/${sha1%$suffix}/$suffix &&
		echo $sha1
	}

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t1450-fsck.sh

@@ -194,6 +194,26 @@ test_expect_success 'tag pointing to something else than its type' '
 	test_must_fail git fsck --tags
 '
 
+test_expect_success 'tag with incorrect tag name & missing tagger' '
+	sha=$(git rev-parse HEAD) &&
+	cat >wrong-tag <<-EOF &&
+	object $sha
+	type commit
+	tag wrong name format
+
+	This is an invalid tag.
+	EOF
+
+	tag=$(git hash-object -t tag -w --stdin <wrong-tag) &&
+	test_when_finished "remove_object $tag" &&
+	echo $tag >.git/refs/tags/wrong &&
+	test_when_finished "git update-ref -d refs/tags/wrong" &&
+	git fsck --tags 2>out &&
+	cat out &&
+	grep "invalid .tag. name" out &&
+	grep "expected .tagger. line" out
+'
+
 test_expect_success 'cleaned up' '
 	git fsck >actual 2>&1 &&
 	test_cmp empty actual