t/test-lib: add an SHA1 prerequisite

There are some basic tests in our codebase that test that we get fixed
SHA-1 values.  These are valuable because they make sure that our SHA-1
implementation is free of bugs, but obviously these tests will fail with
a different hash.

There are also tests which intentionally produce objects that have
collisions when truncated to a certain length to test our handling of
these cases.  These tests, too, will fail with a different hash.

Add an SHA1 prerequisite to annotate both of these types of tests and
disable them when we're using a different hash.  In the future, we will
create versions of these tests which handle both SHA-1 and NewHash.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -1212,3 +1212,10 @@ test_lazy_prereq TIME_T_IS_64BIT 'test-tool date time_t-is64bit'
 test_lazy_prereq CURL '
 	curl --version
 '
+
+# SHA1 is a test if the hash algorithm in use is SHA-1.  This is both for tests
+# which will not work with other hash algorithms and tests that work but don't
+# test anything meaningful (e.g. special values which cause short collisions).
+test_lazy_prereq SHA1 '
+	test $(git hash-object /dev/null) = e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+'