Use less memory in "git log"

This trivially avoids keeping the commit message data around after we
don't need it any more, avoiding a continually growing "git log" memory
footprint.

It's not a huge deal, but it's somewhat noticeable. For the current kernel
tree, doing a full "git log" I got

 - before: /usr/bin/time git log > /dev/null
	0.81user 0.02system 0:00.84elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
	0inputs+0outputs (0major+8851minor)pagefaults 0swaps

 - after: /usr/bin/time git log > /dev/null
	0.79user 0.03system 0:00.83elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
	0inputs+0outputs (0major+5039minor)pagefaults 0swaps

ie the touched pages dropped from 8851 to 5039. For the historic kernel
archive, the numbers are 18357->11037 minor page faults.

We could/should in theory free the commits themselves, but that's really a
lot harder, since during revision traversal we may hit the same commit
twice through different children having it as a parent, even after we've
shown it once (when that happens, we'll silently ignore it next time, but
we still need the "struct commit" to know).

And as the commit message data is clearly the biggest part of the commit,
this is the really easy 60% solution.

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

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