tests: add some script lint checks

tests: add some script lint checks

There are some common but minor errors we tend to make in
writing test scripts:

  1. Scripts are left non-executable. This is not usually
     noticed immediately because "make test" does not need
     the bit, but it is a matter of git policy to make them
     executable (and is a slight convenience when running
     individual scripts).

  2. Two scripts are allocated the same number. Usually this
     happens on separate branches, and the problem only
     comes about during a merge. But since there is no
     textual conflict, the merger would have to be very
     observant to notice.

     This is also a minor error, but can make GIT_SKIP_TESTS
     ambiguous.

This patch introduces a "test-lint" target which checks
both. It is not invoked by default. You can invoke it as
"make test-lint", or you can make it a prerequisite of
running the tests by specifying "TEST_LINT = test-lint" in
your config.mak or on the command line.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>