Optimize the common cases of git-read-tree

Optimize the common cases of git-read-tree

This optimizes bind_merge() and oneway_merge() to not unnecessarily
remove and re-add the old index entries when they can just get replaced
by updated ones.

This makes these operations much faster for large trees (where "large"
is in the 50,000+ file range), because we don't unnecessarily move index
entries around in the index array all the time.

Using the "bummer" tree (a test-tree with 100,000 files) we get:

Before:
	[torvalds@woody bummer]$ time git commit -m"Change one file" 50/500
	real    0m9.470s
	user    0m8.729s
	sys     0m0.476s

After:
	[torvalds@woody bummer]$ time git commit -m"Change one file" 50/500
	real    0m1.173s
	user    0m0.720s
	sys     0m0.452s

so for large trees this is easily very noticeable indeed.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>