t7900: speed up expensive test

A test marked with EXPENSIVE creates two 2.5GB files and adds them to
the repository. This takes 194s to run on my machine, versus 2s when the
EXPENSIVE prereq isn't set. We can trim this down a bit by doing two
things:

  - use "git commit --quiet" to avoid spending time generating a diff
    summary (this actually only helps for the second commit, but I've
    added it here to both for consistency). This shaves off 8s.

  - set core.compression to 0. We know these files are full of random
    bytes, and so won't compress (that's the point of the test!).
    Spending cycles on zlib is pointless. This shaves off 122s.

After this, my total time to run the script is 64s. That won't help
normal runs without GIT_TEST_LONG set, of course, but it's easy enough
to do.

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

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