Make git_dir a path relative to work_tree in setup_work_tree()

Once we find the absolute paths for git_dir and work_tree, we can make
git_dir a relative path since we know pwd will be work_tree. This should
save the kernel some time traversing the path to work_tree all the time
if git_dir is inside work_tree.

Daniel's patch didn't apply for me as-is, so I recreated it with some
differences, and here are the numbers from ten runs each.

There is some IO for me - probably due to more-or-less random flushing of
the journal - so the variation is bigger than I'd like, but whatever:

	Before:
		real    0m8.135s
		real    0m7.933s
		real    0m8.080s
		real    0m7.954s
		real    0m7.949s
		real    0m8.112s
		real    0m7.934s
		real    0m8.059s
		real    0m7.979s
		real    0m8.038s

	After:
		real    0m7.685s
		real    0m7.968s
		real    0m7.703s
		real    0m7.850s
		real    0m7.995s
		real    0m7.817s
		real    0m7.963s
		real    0m7.955s
		real    0m7.848s
		real    0m7.969s

Now, going by "best of ten" (on the assumption that the longer numbers
are all due to IO), I'm saying a 7.933s -> 7.685s reduction, and it does
seem to be outside of the noise (ie the "after" case never broke 8s, while
the "before" case did so half the time).

So looks like about 3% to me.

Doing it for a slightly smaller test-case (just the "arch" subdirectory)
gets more stable numbers probably due to not filling the journal with
metadata updates, so we have:

	Before:
		real    0m1.633s
		real    0m1.633s
		real    0m1.633s
		real    0m1.632s
		real    0m1.632s
		real    0m1.630s
		real    0m1.634s
		real    0m1.631s
		real    0m1.632s
		real    0m1.632s

	After:
		real    0m1.610s
		real    0m1.609s
		real    0m1.610s
		real    0m1.608s
		real    0m1.607s
		real    0m1.610s
		real    0m1.609s
		real    0m1.611s
		real    0m1.608s
		real    0m1.611s

where I'ld just take the averages and say 1.632 vs 1.610, which is just
over 1% peformance improvement.

So it's not in the noise, but it's not as big as I initially thought and
measured.

(That said, it obviously depends on how deep the working directory path is
too, and whether it is behind NFS or something else that might need to
cause more work to look up).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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