test: use --numstat instead of --stat in "git stash show" tests

test: use --numstat instead of --stat in "git stash show" tests

git's diff --stat output is intended for human consumption and
since v1.7.9.2~13 (2012-02-01) varies by locale.  Add a test checking
that git stash show defaults to --stat and tweak the rest of the
"stash show" tests that showed a diffstat to use numstat.

This way, there are fewer tests to tweak if the diffstat format
changes again.  This also improves test coverage when running tests
with git configured not to write its output in the C locale (e.g.,
via GETTEXT_POISON=Yes).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>