t: allow testing different hash algorithms via environment

To allow developers to run the testsuite with a different algorithm than
the default, provide an environment variable, GIT_TEST_DEFAULT_HASH, to
specify the algorithm to use. Compute the fixed constants using
test_oid. Move the constant initialization down below the point where
test-lib-functions.sh is loaded so the functions are defined.

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

t/test-lib-functions.sh

@@ -1417,9 +1417,7 @@ test_set_hash () {
 
 # Detect the hash algorithm in use.
 test_detect_hash () {
-	# Currently we only support SHA-1, but in the future this function will
-	# actually detect the algorithm in use.
-	test_hash_algo='sha1'
+	test_hash_algo="${GIT_TEST_DEFAULT_HASH:-sha1}"
 }
 
 # Load common hash metadata and common placeholder object IDs for use with

t/test-lib.sh

@@ -452,6 +452,9 @@ export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR
 
+GIT_DEFAULT_HASH="${GIT_TEST_DEFAULT_HASH:-sha1}"
+export GIT_DEFAULT_HASH
+
 # Tests using GIT_TRACE typically don't want <timestamp> <file>:<line> output
 GIT_TRACE_BARE=1
 export GIT_TRACE_BARE