In order to selectively skip tests, the environment variable GIT_SKIP_TESTS
can be set like this:

  $ GIT_SKIP_TESTS='t1301 t4150.18' make test

That is, its value can contain only the test script numbers, but not the
full script name. Therefore, it is important that the test scripts are
uniquely numbered. This makes it so.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>