t5310: drop size of truncated ewah bitmap

t5310: drop size of truncated ewah bitmap

We truncate the .bitmap file to 512 bytes and expect to run into
problems reading an individual ewah file. But this length is somewhat
arbitrary, and just happened to work when the test was added in
9d2e330b17 (ewah_read_mmap: bounds-check mmap reads, 2018-06-14).

An upcoming commit will change the size of the history we create in the
test repo, which will cause this test to fail. We can future-proof it a
bit more by reducing the size of the truncated bitmap file.

Signed-off-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>