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>

lib/diff.tcl

@@ -40,6 +40,15 @@ proc reshow_diff {} {
 	}
 }
 
+proc force_diff_encoding {enc} {
+	global current_diff_path
+	
+	if {$current_diff_path ne {}} {
+		force_path_encoding $current_diff_path $enc
+		reshow_diff
+	}
+}
+
 proc handle_empty_diff {} {
 	global current_diff_path file_states file_lists