t5300: choose the built-in hash outside of a repo

Right now, the built-in default hash is always SHA-1, but that will
change in a future commit.  Instead of assuming that operating outside
of a repository will always use SHA-1, look up the default hash
algorithm for operating outside of a repository using an appropriate
environment variable, which will always be correct.

Additionally, for operations outside of a repository, use the
DEFAULT_HASH_ALGORITHM prerequisite rather than SHA1.

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

Author: bk2204

t/t5300-pack-object.sh

@@ -525,7 +525,7 @@ test_expect_success 'index-pack --strict <pack> works in non-repo' '
 	test_path_is_file foo.idx
 '
 
-test_expect_success SHA1 'show-index works OK outside a repository' '
+test_expect_success DEFAULT_HASH_ALGORITHM 'show-index works OK outside a repository' '
 	nongit git show-index <foo.idx
 '
 
@@ -658,7 +658,7 @@ do
 		test_commit -C repo initial &&
 		git -C repo repack -ad &&
 		git -C repo verify-pack "$(pwd)"/repo/.git/objects/pack/*.idx &&
-		if test $hash = sha1
+		if test $hash = $GIT_TEST_BUILTIN_HASH
 		then
 			nongit git verify-pack "$(pwd)"/repo/.git/objects/pack/*.idx
 		else
@@ -676,7 +676,7 @@ do
 		test_commit -C repo initial &&
 		git -C repo repack -ad &&
 		git -C repo index-pack --verify "$(pwd)"/repo/.git/objects/pack/*.pack &&
-		if test $hash = sha1
+		if test $hash = $GIT_TEST_BUILTIN_HASH
 		then
 			nongit git index-pack --verify "$(pwd)"/repo/.git/objects/pack/*.pack
 		else