Makefile: ask "ls-files" to list source files if available

Makefile: ask "ls-files" to list source files if available

The [ce]tags and cscope targets used to run "find" looking for any paths
that match '*.[chS]' to feed the list of source files to downstream xargs.

Use "git ls-files" if it is already available to us, and otherwise use a
tighter "find" expression that does not list directories and does not go
into our .git directory.

Signed-off-by: Junio C Hamano <gitster@pobox.com>