git-gui: add hotkeys to set widget focus

git-gui: add hotkeys to set widget focus

The user cannot change focus between the list of files, the diff view and
the commit message widgets without using the mouse (clicking either of
the four widgets).

With this patch, the user may set ui focus to the previously selected path
in either the "Unstaged Changes" or "Staged Changes" widgets, using
ALT+1 or ALT+2.

The user may also set the ui focus to the diff view widget with
ALT+3, or to the commit message widget with ALT+4.

This enables the user to select/unselect files, view the diff and create a
commit in git-gui using keyboard-only.

Signed-off-by: Birger Skogeng Pedersen <birger.sp@gmail.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>