git-gui: Convert UI to use 'staged for commit' interface.

git-gui: Convert UI to use 'staged for commit' interface.

This is a rather drastic change to the git-gui user interface, but it
doesn't really look any different yet.  I've taken the two lists and
converted them to being "changes to be committed" and "changed but
not updated".  These lists correspond to the same lists output by
git-runstatus based on how files differ in the HEAD<->index and the
index<->working directory comparsions it performs.

This change is meant to correlate with the change in Git 1.5.0 where
we have brought the index more into the foreground and are trying to
teach users to make use of it as part of their daily operations.

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