t: default to compile-time default hash if not set

Right now, the default compile-time hash is SHA-1.  However, in the
future, this might change and it would be helpful to gracefully handle
this case in our testsuite.

To avoid making these assumptions, let's introduce a variable that
contains the built-in default hash and use it in our setup code as the
fallback value if no hash was explicitly set.  For now, this is always
SHA-1, but in a future commit, we'll allow adjusting this and the
variable will be more useful.

To allow us to make our tests more robust, allow test_oid to take the
--hash=builtin option to specify this hash, whatever it is.

Additionally, add a DEFAULT_HASH_ALGORITHM prerequisite to check for the
compile-time hash.

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

t/test-lib.sh

@@ -536,7 +536,8 @@ export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
 export EDITOR
 
-GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-sha1}"
+GIT_TEST_BUILTIN_HASH=sha1
+GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-$GIT_TEST_BUILTIN_HASH}"
 export GIT_DEFAULT_HASH
 GIT_DEFAULT_REF_FORMAT="${GIT_TEST_DEFAULT_REF_FORMAT:-files}"
 export GIT_DEFAULT_REF_FORMAT
@@ -1895,6 +1896,10 @@ test_lazy_prereq SHA1 '
 	esac
 '
 
+test_lazy_prereq DEFAULT_HASH_ALGORITHM '
+	test "$GIT_TEST_BUILTIN_HASH" = "$GIT_DEFAULT_HASH"
+'
+
 test_lazy_prereq DEFAULT_REPO_FORMAT '
 	test_have_prereq SHA1,REFFILES
 '