[PATCH] Improve git-rev-list memory usage further

[PATCH] Improve git-rev-list memory usage further

This avoids keeping tree entries around, and free's them as it traverses
the list. This avoids building up a huge memory footprint just for these
small but very common allocations.

Before:

	$ /usr/bin/time git-rev-list --objects v2.6.12..HEAD | wc -l
	11.65user 0.38system 0:12.65elapsed 95%CPU (0avgtext+0avgdata 0maxresident)k
	0inputs+0outputs (0major+42934minor)pagefaults 0swaps
	59124

After:

	$ /usr/bin/time git-rev-list --objects v2.6.12..HEAD | wc -l
	12.28user 0.29system 0:12.57elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
	0inputs+0outputs (0major+26718minor)pagefaults 0swaps
	59124

Note how the minor fault numbers - which ends up being how many pages we
needed to map - go down from 42934 (167 MB) to 26718 (104 MB).  That is:

Before:
	42934 minor pagefaults

After:

	26718 minor pagefaults

This is all in _addition_ to the previous fixes.  It used to be
~48,000 pagefaults.

That's still a honking big memory footprint, but it's about half of what
it was just a day or two ago (and this is the object list for a pretty big
update - almost 60,000 objects. Smaller updates need less memory).

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