Implement limited context matching in git-apply.

Implement limited context matching in git-apply.

Ok this really should be the good version.  The option
handling has been reworked to be automation safe.

Currently to import the -mm tree I have to work around
git-apply by using patch.  Because some of Andrews
patches in quilt will only apply with fuzz.

I started out implementing a --fuzz option and then I realized
fuzz is not a very safe concept for an automated system.  What
you really want is a minimum number of context lines that must
match.  This allows policy to be set without knowing how many
lines of context a patch actually provides.   By default
the policy remains to match all provided lines of context.

Allowng git-apply to match a restricted set of context makes
it much easier to import the -mm tree into git.  I am still only
processing  1.5 to 1.6 patches a second for the 692 patches in
2.6.17-rc1-mm2 is still painful but it does help.

If I just loop through all of Andrews patches in order
and run git-apply --index -C1 I process the entire patchset
in 1m53s or about 6 patches per second.  So running
git-mailinfo, git-write-tree, git-commit-tree, and
git-update-ref everytime has a measurable impact,
and shows things can be speeded up even more.

All of these timings were taking on my poor 700Mhz Athlon
with 512MB of ram.  So people with fast machiens should
see much better performance.

When a match is found after the number of context are reduced a
warning is generated.  Since this is a rare event and possibly
dangerous this seems to make sense.  Unless you are patching
a single file the error message is a little bit terse at
the moment, but it should be easy to go back and fix.

I have also updated the documentation for git-apply to reflect
the new -C option that sets the minimum number of context
lines that must match.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>