t1301: set umask in reflog sharedrepository=group test

t1301: set umask in reflog sharedrepository=group test

The t1301 script sets the umask globally before many of the
tests. Most of the tests that care about the umask then set
it explicitly at the start of the test. However, one test
does not, and relies on the 077 umask setting from earlier
tests. This is fragile and can break if another test is
added in between. Let's be more explicit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>