unpack: do not mutate cwd when extract strip empties a path

[dyk1454683243-sudo]

Fixes #294

Problem

extract({ strip }) treated a fully-stripped directory entry as the extraction directory. As root (or with preserveOwner / chmod), that applied the archive member's uid/gid, mode, and mtime to an existing cwd.

GNU tar --strip-components skips members whose names become empty. It does not chown or chmod . in the reporter's reproduction:

mkdir dir
sudo chown 501 dir
tar -czf tarball.tgz dir
sudo tar --strip-components=1 -xzf tarball.tgz
# cwd owner unchanged

node-tar's equivalent tar.x({ file, strip: 1 }) changed cwd ownership to 501.

This matches the existing contract in TarOptions.strip ("any entry whose entire path is stripped will be excluded"), but dir and dir/ still resolved to cwd after parts.splice.

Fix

After applying strip, skip the entry when only empty / . segments remain. Remaining names (pkg/sub → sub) are still extracted, including metadata on existing remaining directories, which is what GNU tar -p --strip-components does.

Tests

test/unpack.js (strip does not mutate existing cwd (#294)):

• dir-only archive with and without trailing slash (sync + async)
• preserveOwner does not chown cwd
• existing remaining dir still gets mode after strip
• without strip, the directory entry is still extracted onto dir/ (cwd unchanged)
• ./keepme after strip: 1 still extracts
• strip: 2 on a/b/ does not mutate cwd
• pre/. after strip: 1 does not mutate cwd

Discriminating check: the new tests fail on 2a22bfc (17 assertion failures) and pass on this branch.

Test plan

• Reproduced against GNU tar 1.35 vs node-tar 7.5.22 (unfixed: cwd mode 755→700; as root, cwd uid→501)
• After the fix, GNU tar and node-tar agree: cwd unchanged; remaining existing dirs still get metadata
• sudo reproduction: cwd uid stays 1000 when archive dir is uid 501
• npx tap test/unpack.js --disable-coverage -t0: 692/692 pass
• New tests fail without the CHECKPATH skip and pass with it