t3903: stop hard-coding commit sha1s

When testing the diff output of "git stash list", we look
for the stash's subject of "WIP on master: $sha1", even
though it's not relevant to the diff output. This makes the
test brittle to refactoring, as any changes to earlier tests
may impact the commit sha1.

Since we don't care about the commit subject here, we can
simply ask stash not to print it.

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