Clean up git-ls-file directory walking library interface

Clean up git-ls-file directory walking library interface

This moves the code to add the per-directory ignore files for the base
directory into the library routine.

That not only allows us to turn the function push_exclude_per_directory()
static again, it also simplifies the library interface a lot (the caller
no longer needs to worry about any of the per-directory exclude files at
all).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>