t/t5800: skip if python version is older than 2.5

This test script depends on the git-remote-testgit python script.  This
python script makes use of the hashlib module which was released in python
version 2.5.  So, add a new pre-requisite named PYTHON_2_5_OR_NEWER to
test-lib.sh and check for it in t5800.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5800-remote-helpers.sh

@@ -7,9 +7,15 @@ test_description='Test remote-helper import and export commands'
 
 . ./test-lib.sh
 
-if ! test_have_prereq PYTHON
+if test_have_prereq PYTHON && "$PYTHON_PATH" -c '
+import sys
+if sys.hexversion < 0x02050000:
+    sys.exit(1)
+'
 then
-	say 'skipping git remote-testgit tests: requires Python support'
+	:
+else
+	say 'skipping git remote-testgit tests: requires Python 2.5 or newer'
 	test_done
 fi