git-gui: Created edit menu and basic editing bindings.

git-gui: Created edit menu and basic editing bindings.

Users have come to expect basic editing features within their
applications, such as cut/copy/paste/undo/redo/select-all.  I
found these features to be lacking in git-gui so now we have
them.

I also added basic keyboard bindings for the diff viewing area
so that the arrow keys move around single units (lines or columns)
and the M1-X/C keys will copy the selected text and the M1-A key
will select the entire diff.

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