setup.c: document get_pathspec()
setup.c: document get_pathspec()

Since we have just created a new pathspec-handling library, now is a
good time to add some comments explaining get_pathspec().

Signed-off-by: Adam Spiers <46f1a0bd5592a2f9244ca321b129902a06b53e03@adamspiers.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git