Makefile: exclude contrib from FIND_SOURCE_FILES

Makefile: exclude contrib from FIND_SOURCE_FILES

When you're working on the git project, you're unlikely to
care about random bits in contrib/ (e.g., you would not want
to jump to the copy of xmalloc in the wincred credential
helper). Nobody has really complained because there are
relatively few C files in contrib.

Now that we're matching shell scripts, too, we get quite a
few more hits, especially in the obsolete contrib/examples
directory. Looking for usage() should turn up the one in
git-sh-setup, not in some long-dead version of git-clone.

Let's just exclude all of contrib. Any specific projects
there which are big enough to want tags can generate them
separately.

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