t5310-pack-bitmaps: skip JGit tests with SHA256

t5310-pack-bitmaps: skip JGit tests with SHA256

In 't5310-pack-bitmaps.sh' two tests make sure that our pack bitmaps
are compatible with JGit's bitmaps.  Alas, not even the most recent
JGit version (5.9.0.202009080501-r) supports SHA256 yet, so when this
test script is run with GIT_TEST_DEFAULT_HASH=sha256 on a setup with
JGit installed in PATH, then these two tests fail.

Protect these two tests with the SHA1 prereq in order to skip them
when testing with SHA256.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>