Makefile: exclude test cruft from FIND_SOURCE_FILES

Makefile: exclude test cruft from FIND_SOURCE_FILES

The test directory may contain three types of files that
match our patterns:

  1. Helper programs in t/helper.

  2. Sample data files (e.g., t/t4051/hello.c).

  3. Untracked cruft in trash directories and t/perf/build.

We want to match (1), but not the other two, as they just
clutter up the list.

For the ls-files method, we can drop (2) with a negative
pathspec. We do not have to care about (3), since ls-files
will not list untracked files.

For `find`, we can match both cases with `-prune` patterns.

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