[PATCH 5/6] Make cg-diff use optparse, and add features

This adds some new features to cg-diff:

  - diffstat (or rather git-apply --stat) support. The
    "-d" option outputs (optionally colorized) diff stats
    before the diff.

  - support for more git-diff-[tree/cache] options:
    -B, -R, -M, -C are now all passed on.

  - The COGITO_AUTO_COLOR environment variable makes output
    automatically colorized, if set and if we're on a color
    capable terminal.

We also use the new optparse function from cg-Xlib, to allow
for e.g. "cg-diff -drorigin" and stuff.

Reuse colorization logic from cg-Xlib.

And use LESS to make "less" search for chunks and diff --git
markers. This allows you to "n" your way through a series
of diffs, and gives a nice visual separation of patches.

Signed-off-by: Dan Holmsand <redacted>
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