fix(tag): split tag object headers at line feeds only

[Keerthana-64]

TagObject._set_cache_ split the decoded tag object with str.splitlines() and then read the headers by position, taking lines[3] as the tagger. str.splitlines() also breaks at \r, \v, \f, \x1c-\x1e, \x85, U+2028 and U+2029, but git ends a header only at a line feed. A U+2028 inside the tag header of an untrusted tag object therefore shifts every later header by one line in GitPython only. With tag v1<U+2028>tagger Linus Torvalds <torvalds@linux-foundation.org> 1700000000 +0000 followed by a real tagger Mallory <m@evil.example> ... line, git cat-file tag, git show and git for-each-ref --format=%(taggername) report Mallory, while GitPython reported tag == "v1" and Linus Torvalds as the tagger. The object passes git fsck --strict and a clone with transfer.fsckObjects=true, because git only warns badTagName.

The fix splits at "\n" and drops the trailing empty element, so tags that only use line feeds parse exactly as before. A \r or U+2028 inside the message is now kept, as git shows it. The new test in test/test_refs.py fails before the change and passes after it. ruff, ruff format --check and mypy are clean. On my machine the full suite has 3 failures (test_empty_repo, test_rebasing, test_diff_with_staged_file), and all 3 also fail without this change.