git-gui: Allow forcing display encoding for diffs using a submenu.

git-gui: Allow forcing display encoding for diffs using a submenu.

Add a submenu to allow dynamically changing the encoding to use
for diffs. Encoding settings are remembered while git-gui runs.
The rules are:

1) Encoding set for a specific file overrides gitattributes.
2) Last explicitly set value of the encoding overrides gui.encoding

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Tested-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>