Dynamically adjusting packed_git_window_size

Internally we are using a custom build of Git, and one of the patches that I apply to newer builds of Git is one to adjust the DEFAULT_PACKED_GIT_WINDOW_SIZE in git-compat-util.h so Git won't trample all over our ulimit values on the 64-bit dev machines.

To do away with this, I've got these two (really one) set of patches to adjust the packed_git_window_size when setup_git_env() is called to a fraction of the "addressspace" limit (RLIMIT_AS). If the user's environment defines "ulimit -v" as "unlimited", this code will not take effect.

It's worth noting that this doesn't force Git to respect these limits, I'm still tracking down an issue hiding in get_revision() where I'm experiencing mmap(2) failures executing a `git log` command with restrictive ulimit settings (Linus, since you were so "pleased" with my last epic gdb fail, here's today's):

	(gdb)
	open_packed_git (p=0x71f2e0) at sha1_file.c:733
	733             /* We leave these file descriptors open with sliding mmap;
	(gdb)
	735              */
	(gdb)
	741                     return error("cannot set FD_CLOEXEC");
	(gdb)
	746             if (hdr.hdr_signature != htonl(PACK_SIGNATURE))
	(gdb)

	Recursive internal problem.
	[1]    17381 abort      GIT_PAGER= gdb git
	tyler@starfruit:~/source/git/main>

Oi vei.

Cheers, -R. Tyler Ballance