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>