Create pathname-based hash-table lookup into index

This creates a hash index of every single file added to the index.
Right now that hash index isn't actually used for much: I implemented a
"cache_name_exists()" function that uses it to efficiently look up a
filename in the index without having to do the O(logn) binary search,
but quite frankly, that's not why this patch is interesting.

No, the whole and only reason to create the hash of the filenames in the
index is that by modifying the hash function, you can fairly easily do
things like making it always hash equivalent names into the same bucket.

That, in turn, means that suddenly questions like "does this name exist
in the index under an _equivalent_ name?" becomes much much cheaper.

Guiding principles behind this patch:

 - it shouldn't be too costly. In fact, my primary goal here was to
   actually speed up "git commit" with a fully populated kernel tree, by
   being faster at checking whether a file already existed in the index. I
   did succeed, but only barely:

	Best before:
		[torvalds@woody linux]$ time git commit > /dev/null
		real    0m0.255s
		user    0m0.168s
		sys     0m0.088s

	Best after:

		[torvalds@woody linux]$ time ~/git/git commit > /dev/null
		real    0m0.233s
		user    0m0.144s
		sys     0m0.088s

   so some things are actually faster (~8%).

   Caveat: that's really the best case. Other things are invariably going
   to be slightly slower, since we populate that index cache, and quite
   frankly, few things really use it to look things up.

   That said, the cost is really quite small. The worst case is probably
   doing a "git ls-files", which will do very little except puopulate the
   index, and never actually looks anything up in it, just lists it.

	Before:
		[torvalds@woody linux]$ time git ls-files > /dev/null
		real    0m0.016s
		user    0m0.016s
		sys     0m0.000s

	After:
		[torvalds@woody linux]$ time ~/git/git ls-files > /dev/null
		real    0m0.021s
		user    0m0.012s
		sys     0m0.008s

   and while the thing has really gotten relatively much slower, we're
   still talking about something almost unmeasurable (eg 5ms). And that
   really should be pretty much the worst case.

   So we lose 5ms on one "benchmark", but win 22ms on another. Pick your
   poison - this patch has the advantage that it will _likely_ speed up
   the cases that are complex and expensive more than it slows down the
   cases that are already so fast that nobody cares. But if you look at
   relative speedups/slowdowns, it doesn't look so good.

 - It should be simple and clean

   The code may be a bit subtle (the reasons I do hash removal the way I
   do etc), but it re-uses the existing hash.c files, so it really is
   fairly small and straightforward apart from a few odd details.

Now, this patch on its own doesn't really do much, but I think it's worth
looking at, if only because if done correctly, the name hashing really can
make an improvement to the whole issue of "do we have a filename that
looks like this in the index already". And at least it gets real testing
by being used even by default (ie there is a real use-case for it even
without any insane filesystems).

NOTE NOTE NOTE! The current hash is a joke. I'm ashamed of it, I'm just
not ashamed of it enough to really care. I took all the numbers out of my
nether regions - I'm sure it's good enough that it works in practice, but
the whole point was that you can make a really much fancier hash that
hashes characters not directly, but by their upper-case value or something
like that, and thus you get a case-insensitive hash, while still keeping
the name and the index itself totally case sensitive.

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

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