This changes the way how pathspec is used in the three diff-*
brothers.  Earlier, they tried to grab as much information from
the original input and used pathspec to limit the output.  This
version uses pathspec upfront to narrow the world diffcore
operates in, so "git-diff-* <arguments> some-directory" does not
look at things outside the specified subtree when finding
rename/copy or running pickaxe.

Since diff-tree already takes this view and does not feed
anything outside the specified directotires to begin with, this
patch does not have to touch that command.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>