t5400: explicitly use "gc" strategy

t5400: explicitly use "gc" strategy

In t5400 we verify that git-receive-pack(1) runs automated repository
maintenance in the remote repository. The check is performed indirectly
by observing an effect that git-gc(1) would have, namely to prune a
temporary object from the object database. In a subsequent commit we're
about to switch to the "geometric" strategy by default though, and here
we stop observing that effect.

Adapt the test to explicitly use the "gc" strategy to prepare for that
upcoming change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>