git-gui: Additional early feature development.

* Run refresh before diff-index.
* Load saved commit message during rescan.
* Save current commit message (if any) during quit.
* Add Signed-off-by line to commit buffer.
* Batch update-index invocations through --stdin.
* Better highlight which file is in the diff viewer.
* Key bindings for signoff, check-in all and commit.
* Improved formatting of status table within source.

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

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>