Some more memory leak avoidance

Some more memory leak avoidance

This is really the dregs of my effort to not waste memory in git-rev-list,
and makes barely one percent of a difference in the memory footprint, but
hey, it's also a pretty small patch.

It discards the parent lists and the commit buffer after the commit has
been shown by git-rev-list (and "git log" - which already did the commit
buffer part), and frees the commit list entry that was used by the
revision walker.

The big win would be to get rid of the "refs" pointer in the object
structure (another 5%), because it's only used by fsck. That would require
some pretty major surgery to fsck, though, so I'm timid and did the less
interesting but much easier part instead.

This (percentually) makes a bigger difference to "git log" and friends,
since those are walking _just_ commits, and thus the list entries tend to
be a bigger percentage of the memory use. But the "list all objects" case
does improve too.

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