add.c: extract check_path_for_gitlink() from treat_gitlinks() for reuse

Extract the body of the for loop in treat_gitlinks() into a separate
check_path_for_gitlink() function so that it can be reused elsewhere.
This paves the way for a new check-ignore sub-command.

Also document treat_gitlinks().

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>