Fix testcase failure when extended attributes are in use

Fix testcase failure when extended attributes are in use

06cbe855 (Make core.sharedRepository more generic, 2008-04-16) made
several testcases in t1301-shared-repo.sh which fail if on a system
which creates files with extended attributes (e.g. SELinux), since ls
appends a '+' sign to the permission set in such cases.  In fact,
POSIX.1 allows ls to add a single printable character after the usual
3x3 permission bits to show that an optional alternate/additional access
method is associated with the path.

This fixes the testcase to strip any such sign prior to verifying the
permission set.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Tested-by: Deskin Miller <deskinm@umich.edu>