tests: suppress system gitattributes

Set GIT_ATTR_NOSYSTEM in test-lib to make tests more reliable in two
ways:

 - an invalid GIT_ATTR_NOSYSTEM setting should not cause tests to fail
   with "fatal: bad config value for 'GIT_ATTR_NOSYSTEM'".

 - /etc/gitattributes should not change the outcome of tests.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -948,7 +948,8 @@ GIT_TEMPLATE_DIR="$GIT_BUILD_DIR"/templates/blt
 unset GIT_CONFIG
 GIT_CONFIG_NOSYSTEM=1
 GIT_CONFIG_NOGLOBAL=1
-export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_CONFIG_NOGLOBAL
+GIT_ATTR_NOSYSTEM=1
+export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG_NOSYSTEM GIT_CONFIG_NOGLOBAL GIT_ATTR_NOSYSTEM
 
 . "$GIT_BUILD_DIR"/GIT-BUILD-OPTIONS