Remove diff machinery dependency from read-cache

Exal Sibeaz pointed out that some git files are way too big, and that
add_files_to_cache() brings in all the diff machinery to any git binary
that needs the basic git SHA1 object operations from read-cache.c. Which
is pretty much all of them.

It's doubly silly, since add_files_to_cache() is only used by builtin
programs (add, checkout and commit), so it's fairly easily fixed by just
moving the thing to builtin-add.c, and avoiding the dependency entirely.

I initially argued to Exal that it would probably be best to try to depend
on smart compilers and linkers, but after spending some time trying to
make -ffunction-sections work and giving up, I think Exal was right, and
the fix is to just do some trivial cleanups like this.

This trivial cleanup results in pretty stunning file size differences.
The diff machinery really is mostly used by just the builtin programs, and
you have things like these trivial before-and-after numbers:

  -rwxr-xr-x 1 torvalds torvalds 1727420 2010-01-21 10:53 git-hash-object
  -rwxrwxr-x 1 torvalds torvalds  940265 2010-01-21 11:16 git-hash-object

Now, I'm not saying that 940kB is good either, but that's mostly all the
debug information - you can see the real code with 'size':

   text	   data	    bss	    dec	    hex	filename
 418675	   3920	 127408	 550003	  86473	git-hash-object (before)
 230650	   2288	 111728	 344666	  5425a	git-hash-object (after)

ie we have a nice 24% size reduction from this trivial cleanup.

It's not just that one file either. I get:

	[torvalds@nehalem git]$ du -s /home/torvalds/libexec/git-core
	45640	/home/torvalds/libexec/git-core (before)
	33508	/home/torvalds/libexec/git-core (after)

so we're talking 12MB of diskspace here.

(Of course, stripping all the binaries brings the 33MB down to 9MB, so the
whole debug information thing is still the bulk of it all, but that's a
separate issue entirely)

Now, I'm sure there are other things we should do, and changing our
compiler flags from -O2 to -Os would bring the text size down by an
additional almost 20%, but this thing Exal pointed out seems to be some
good low-hanging fruit.

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

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