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>

t/t5310-pack-bitmaps.sh

@@ -277,7 +277,7 @@ test_expect_success 'pack with missing parent' '
 	git pack-objects --stdout --revs <revs >/dev/null
 '
 
-test_expect_success JGIT 'we can read jgit bitmaps' '
+test_expect_success JGIT,SHA1 'we can read jgit bitmaps' '
 	git clone --bare . compat-jgit.git &&
 	(
 		cd compat-jgit.git &&
@@ -287,7 +287,7 @@ test_expect_success JGIT 'we can read jgit bitmaps' '
 	)
 '
 
-test_expect_success JGIT 'jgit can read our bitmaps' '
+test_expect_success JGIT,SHA1 'jgit can read our bitmaps' '
 	git clone --bare . compat-us.git &&
 	(
 		cd compat-us.git &&