GIT_SKIP_TESTS: allow users to omit tests that are known to break

GIT_SKIP_TESTS: allow users to omit tests that are known to break

In some environments, certain tests have no way of succeeding
due to platform limitation, such as lack of 'unzip' program, or
filesystem that do not allow arbitrary sequence of non-NUL bytes
as pathnames.

You should be able to say something like

	$ cd t
	$ GIT_SKIP_TESTS=t9200.8 t9200-git-cvsexport-commit.sh

and even:

	$ GIT_SKIP_TESTS='t[0-4]??? t91?? t9200.8' make test

to omit such tests.  The value of the environment variable is a
SP separated list of patterns that tells which tests to skip,
and either can match the "t[0-9]{4}" part to skip the whole
test, or t[0-9]{4} followed by ".$number" to say which
particular test to skip.

Note that some tests in the existing test suite rely on previous
test item, so you cannot arbitrarily disable one and expect the
remainder of test to check what the test originally was intended
to check.

Signed-off-by: Junio C Hamano <junkio@cox.net>