Version of dirdiff to display diffs between git trees

[Paul Mackerras <paulus@samba.org>]

I have adapted my "dirdiff" program to be able to display differences
between git trees (without checking them out).  It can display the
differences between up to 5 trees, each of which can be either a git
tree (identified by a tree id, a commit id or a tag) or the working
directory.

The main window summarizes which files have changed.  Double-clicking
on a file name pops up a diff viewer window which displays the
differences between the different versions of the file in a
color-coded unidiff format.  I find it useful to copy .git/HEAD to
.git/OLD_HEAD before doing an update on my linux-2.6 repository, and
then after the update do

	girdiff OLD_HEAD HEAD

to see what has changed.  With the file list I can easily skip over
files that I'm not interested in and then use the diff viewer to look
at changes to files that I do care about.

I have called this version "girdiff" to distinguish it from my normal
dirdiff releases.  It is available at:

	http://ozlabs.org/~paulus/girdiff

If invoked as girdiff, or if invoked as dirdiff and the -g flag is
given, it will interpret its arguments as git trees rather than
directory or file names.  As a special case, the working directory is
indicated by ".".

This is still under development and there are probably bugs - let me
know if you find any.

Paul.