tests: suppress system gitattributes
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 <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Improved-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git