t5000: test long filenames

t5000: test long filenames

Add a file with a long name to the test archive in order to check
entries with pax extended headers.  Also add a check for tar versions
that doen't understand this format.  Those versions should extract the
headers as a regular files.  Add code to check_tar() to interpret the
path header if present, so that our tests work even with those tar
versions.

It's important to use the fallback code only if needed to still be
able to detect git archive errorously creating pax headers as regular
file entries (with a suitable tar version, of course).

The archive used to check for pax header support in tar was generated
using GNU tar 1.26 and its option --format=pax.

Tested successfully on NetBSD 6.1, which has a tar version lacking pax
header support.

Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>