git-gui: Remove TODO list.

I'm apparently not very good at keeping my own TODO file current.
I its also somewhat strange to keep the TODO list as part of the
software branch, as its meta-information that is not directly
related to the code.  I'm pulling the TODO list from git-gui and
moving it into a seperate branch.

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