[LHF] making t5000 "tar xf" tests more lenient

I've been running testsuite on a few platforms that are unfamiliar to me, and was bitten by BSD implementation of tar that do not grok the extended pax headers. I've already fixed one in t9502 [*1*] where we produce a tarball with "git archive" and then try to validate it with the platform implementation of "tar" so that the test won't barf when it sees the extended pax header.

t5000 is littered with similar tests that break unnecessarily with BSD implementations. I think what it wants to test are:

 - "archive" produces tarball, and "tar" can extract from it.  If
   your "tar" does not understand pax header, you may get it as an
   extra file that shouldn't be there, but that should not cause the
   test to fail---in real life, people without a pax-aware "tar"
   will just ignore and remove the header and can go on.

 - "get-tar-commmit-id" can inspect a tarball produced by "archive"
   to recover the object name of the commit even on a platform
   without a pax-aware "tar".

Perhaps t5000 can be restructured like so:

 - create a tarball with the commit-id and test with
   "get-tar-commit-id" to validate it; also create a tarball out of
   a tree to make sure it does not have commit-id and check with
   "get-tar-commit-id".  Do this on any and all platform, even on
   the ones without a pax-aware "tar".

 - check platform implementation of "tar" early to see if extracting
   a simple output from "git archive" results in an extra pax header
   file.  If so, remember this fact and produce any and all tarballs
   used in the remainder of the test by forcing ^{tree}.

so that people on platforms without pax-aware "tar" do not have to install GNU tar only to pass this test.

It would be a good exercise during the holiday week for somebody on BSD (it seems NetBSD is more troublesome than OpenBSD) to come up with a patch to help users on these platforms.

Thanks.

[Footnote]

*1* http://thread.gmane.org/gmane.comp.version-control.git/211803