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