gh-149980: Strip all trailing slashes from GNU long directory names i…

[ankhikarmakar]

Title:
gh-149980: Strip all trailing slashes from GNU long directory names in tarfile

Body:
In tarfile._proc_gnulong, GNU long-name directory entries had only a single
trailing slash removed (removesuffix("/")), while short-name directories and
PAX entries have all trailing slashes stripped (rstrip("/") in _frombuf /
_proc_builtin). This made a foo/// directory show up as foo via the
short path and foo// via the GNU long-name path.

This PR aligns _proc_gnulong with the other paths by using rstrip("/"), and
adds a regression test that writes a GNU-format archive containing both a short
and a long directory name ending in /// and asserts the parsed names match.

• Issue: tarfile: _proc_gnulong uses removesuffix("/") but comment claims consistency with frombuf() which uses rstrip("/") #149980

[python-cla-bot]

All commit authors signed the Contributor License Agreement.