Makefile: strip leading ./ in $(FIND_SOURCE_FILES)

Makefile: strip leading ./ in $(FIND_SOURCE_FILES)

Currently, $(FIND_SOURCE_FILES) has two modes: if `git ls-files` is
present, it will use that to enumerate the files in the repository; else
it will use `$(FIND) .` to enumerate the files in the directory.

There is a subtle difference between these two methods, however. With
ls-files, filenames don't have a leading `./` while with $(FIND), they
do. This does not currently pose a problem but in a future patch, we
will be using `filter-out` to process the list of files with the
assumption that there is no prefix.

Unify the two possible invocations in $(FIND_SOURCE_FILES) by using sed
to remove the `./` prefix in the $(FIND) case.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>