Do exact rename detection regardless of rename limits

Do exact rename detection regardless of rename limits

Now that the exact rename detection is linear-time (with a very small
constant factor to boot), there is no longer any reason to limit it by
the number of files involved.

In some trivial testing, I created a repository with a directory that
had a hundred thousand files in it (all with different contents), and
then moved that directory to show the effects of renaming 100,000 files.

With the new code, that resulted in

	[torvalds@woody big-rename]$ time ~/git/git show -C | wc -l
	400006

	real    0m2.071s
	user    0m1.520s
	sys     0m0.576s

ie the code can correctly detect the hundred thousand renames in about 2
seconds (the number "400006" comes from four lines for each rename:

	diff --git a/really-big-dir/file-1-1-1-1-1 b/moved-big-dir/file-1-1-1-1-1
	similarity index 100%
	rename from really-big-dir/file-1-1-1-1-1
	rename to moved-big-dir/file-1-1-1-1-1

and the extra six lines is from a one-liner commit message and all the
commit information and spacing).

Most of those two seconds weren't even really the rename detection, it's
really all the other stuff needed to get there.

With the old code, this wouldn't have been practically possible.  Doing
a pairwise check of the ten billion possible pairs would have been
prohibitively expensive.  In fact, even with the rename limiter in
place, the old code would waste a lot of time just on the diff_filespec
checks, and despite not even trying to find renames, it used to look
like:

	[torvalds@woody big-rename]$ time git show -C | wc -l
	1400006

	real    0m12.337s
	user    0m12.285s
	sys     0m0.192s

ie we used to take 12 seconds for this load and not even do any rename
detection! (The number 1400006 comes from fourteen lines per file moved:
seven lines each for the delete and the create of a one-liner file, and
the same extra six lines of commit information).

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