Teach git-rev-list to follow just a specified set of files

Teach git-rev-list to follow just a specified set of files

This is the first cut at a git-rev-list that knows to ignore commits that
don't change a certain file (or set of files).

NOTE! For now it only prunes _merge_ commits, and follows the parent where
there are no differences in the set of files specified. In the long run,
I'd like to make it re-write the straight-line history too, but for now
the merge simplification is much more fundamentally important (the
rewriting of straight-line history is largely a separate simplification
phase, but the merge simplification needs to happen early if we want to
optimize away unnecessary commit parsing).

If all parents of a merge change some of the files, the merge is left as
is, so the end result is in no way guaranteed to be a linear history, but
it will often be a lot /more/ linear than the full tree, since it prunes
out parents that didn't matter for that set of files.

As an example from the current kernel:

	[torvalds@g5 linux]$ git-rev-list HEAD | wc -l
	9885
	[torvalds@g5 linux]$ git-rev-list HEAD -- Makefile | wc -l
	4084
	[torvalds@g5 linux]$ git-rev-list HEAD -- drivers/usb | wc -l
	5206

and you can also use 'gitk' to more visually see the pruning of the
history tree, with something like

	gitk -- drivers/usb

showing a simplified history that tries to follow the first parent in a
merge that is the parent that fully defines drivers/usb/.

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