dir.c: clean up handling of 'path' parameter in read_directory_recursive()

dir.c: clean up handling of 'path' parameter in read_directory_recursive()

Right now we pass two different pathnames ('path' and 'base') down to
read_directory_recursive(), and the only real reason for that is that we
want to allow an empty 'base' parameter, but when we do so, we need the
pathname to "opendir()" to be "." rather than the empty string.

And rather than handle that confusion in the caller, we can just fix
read_directory_recursive() to handle the case of an empty path itself,
by just passing opendir() a "." ourselves if the path is empty.

This would allow us to then drop one of the pathnames entirely from the
calling convention, but rather than do that, we'll start separating them
out as a "filesystem pathname" (the one we use for filesystem accesses)
and a "git internal base name" (which is the name that we use for git
internally).

That will eventually allow us to do things like handle different
encodings (eg the filesystem pathnames might be Latin1, while git itself
would use UTF-8 for filename information).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>