Makefile: match shell scripts in FIND_SOURCE_FILES

Makefile: match shell scripts in FIND_SOURCE_FILES

We feed FIND_SOURCE_FILES to ctags to help developers
navigate to particular functions, but we only feed C source
code. The same feature can be helpful when working with
shell scripts (especially the test suite). Modern versions
of ctags know how to parse shell scripts; we just need to
feed the filenames to it.

This patch specifically avoids including the individual test
scripts themselves. Those are unlikely to be of interest,
and there are a lot of them to process. It does pick up
test-lib.sh and test-lib-functions.sh.

Note that our negative pathspec already excludes the
individual scripts for the ls-files case, but we need to
loosen the `find` rule to match it.

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