git-debpush: avoid a pipefail problem in get_file_from_ref

git-debpush: avoid a pipefail problem in get_file_from_ref

`grep -q` exits as soon as it finds a matching line, potentially
sending a SIGPIPE to git-ls-tree.  We have pipefail turned on, so that
can make the whole pipeline exit nonzero, which is wrong when grep did
in fact find a match.

This solution is more readable than disabling pipefail just for this
line (as is done elsewhere in git-debpush).

Closes: #940588
Reported-by: Andrej Shadura <andrewsh@debian.org>
Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>