Add "--dirstat" for some directory statistics

Add "--dirstat" for some directory statistics

This adds a new form of overview diffstat output, doing something that I
have occasionally ended up doing manually (and badly, because it's
actually pretty nasty to do), and that I think is very useful for an
project like the kernel that has a fairly deep and well-separated
directory structure with semantic meaning.

What I mean by that is that it's often interesting to see exactly which
sub-directories are impacted by a patch, and to what degree - even if you
don't perhaps care so much about the individual files themselves.

What makes the concept more interesting is that the "impact" is often
hierarchical: in the kernel, for example, something could either have a
very localized impact to "fs/ext3/" and then it's interesting to see that
such a patch changes mostly that subdirectory, but you could have another
patch that changes some generic VFS-layer issue which affects _many_
subdirectories that are all under "fs/", but none - or perhaps just a
couple of them - of the individual filesystems are interesting in
themselves.

So what commonly happens is that you may have big changes in a specific
sub-subdirectory, but still also significant separate changes to the
subdirectory leading up to that - maybe you have significant VFS-level
changes, but *also* changes under that VFS layer in the NFS-specific
directories, for example. In that case, you do want the low-level parts
that are significant to show up, but then the insignificant ones should
show up as under the more generic top-level directory.

This patch shows all of that with "--dirstat". The output can be either
something simple like

        commit 81772fe...
        Author: Thomas Gleixner <tglx@linutronix.de>
        Date:   Sun Feb 10 23:57:36 2008 +0100

            x86: remove over noisy debug printk

            pageattr-test.c contains a noisy debug printk that people reported.
            The condition under which it prints (randomly tapping into a mem_map[]
            hole and not being able to c_p_a() there) is valid behavior and not
            interesting to report.

            Remove it.

            Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
            Acked-by: Ingo Molnar <mingo@elte.hu>
            Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
         100.0% arch/x86/mm/

or something much more complex like

        commit e231c2e...
        Author: David Howells <dhowells@redhat.com>
        Date:   Thu Feb 7 00:15:26 2008 -0800

            Convert ERR_PTR(PTR_ERR(p)) instances to ERR_CAST(p)

	  20.5% crypto/
	   7.6% fs/afs/
	   7.6% fs/fuse/
	   7.6% fs/gfs2/
	   5.1% fs/jffs2/
	   5.1% fs/nfs/
	   5.1% fs/nfsd/
	   7.6% fs/reiserfs/
	  15.3% fs/
	   7.6% net/rxrpc/
	  10.2% security/keys/

where that latter example is an example of significant work in some
individual fs/*/ subdirectories (like the patches to reiserfs accounting
for 7.6% of the whole), but then discounting those individual filesystems,
there's also 15.3% other "random" things that weren't worth reporting on
their oen left over under fs/ in general (either in that directory itself,
or in subdirectories of fs/ that didn't have enough changes to be reported
individually).

I'd like to stress that the "15.3% fs/" mentioned above is the stuff that
is under fs/ but that was _not_ significant enough to report on its own.
So the above does _not_ mean that 15.3% of the work was under fs/ per se,
because that 15.3% does *not* include the already-reported 7.6% of afs,
7.6% of fuse etc.

If you want to enable "cumulative" directory statistics, you can use the
"--cumulative" flag, which adds up percentages recursively even when
they have been already reported for a sub-directory.  That cumulative
output is disabled if *all* of the changes in one subdirectory come from
a deeper subdirectory, to avoid repeating subdirectories all the way to
the root.

For an example of the cumulative reporting, the above commit becomes

	commit e231c2e...
	Author: David Howells <dhowells@redhat.com>
	Date:   Thu Feb 7 00:15:26 2008 -0800

	    Convert ERR_PTR(PTR_ERR(p)) instances to ERR_CAST(p)

	  20.5% crypto/
	   7.6% fs/afs/
	   7.6% fs/fuse/
	   7.6% fs/gfs2/
	   5.1% fs/jffs2/
	   5.1% fs/nfs/
	   5.1% fs/nfsd/
	   7.6% fs/reiserfs/
	  61.5% fs/
	   7.6% net/rxrpc/
	  10.2% security/keys/

in which the commit percentages now obviously add up to much more than
100%: now the changes that were already reported for the sub-directories
under fs/ are then cumulatively included in the whole percentage of fs/
(ie now shows 61.5% as opposed to the 15.3% without the cumulative
reporting).

The default reporting limit has been arbitrarily set at 3%, which seems
to be a pretty good cut-off, but you can specify the cut-off manually by
giving it as an option parameter (eg "--dirstat=5" makes the cut-off be
at 5% instead)

NOTE! The percentages are purely about the total lines added and removed,
not anything smarter (or dumber) than that. Also note that you should not
generally expect things to add up to 100%: not only does it round down, we
don't report leftover scraps (they add up to the top-level change count,
but we don't even bother reporting that, it only reports subdirectories).

Quite frankly, as a top-level manager this is really convenient for me,
but it's going to be very boring for git itself since there are few
subdirectories. Also, don't expect things to make tons of sense if you
combine this with "-M" and there are cross-directory renames etc.

But even for git itself, you can get some fun statistics. Try out

        git log --dirstat

and see the occasional mentions of things like Documentation/, git-gui/,
gitweb/ and gitk-git/. Or try out something like

        git diff --dirstat v1.5.0..v1.5.4

which does kind of git an overview that shows *something*. But in general,
the output is more exciting for big projects with deeper structure, and
doing a

        git diff --dirstat v2.6.24..v2.6.25-rc1

on the kernel is what I actually wrote this for!

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