git-gui: Initial revision.

git-gui: Initial revision.

This is based on Paul Mackerras' gitool prototype which he offered up
to the community earlier in 2006.  Its mostly however a rewrite from
scratch of a Tcl/Tk based graphical interface for Git and the most
common commands users might need to perform.

Currently it can display the status of the current repository, and not
much else.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>