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>

git-gui.sh

@@ -3010,6 +3010,14 @@ proc create_common_diff_popup {ctxm} {
 		-command {incr_font_size font_diff 1}
 	lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
 	$ctxm add separator
+	set emenu $ctxm.enc
+	menu $emenu
+	build_encoding_menu $emenu [list force_diff_encoding]
+	$ctxm add cascade \
+		-label [mc "Encoding"] \
+		-menu $emenu
+	lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
+	$ctxm add separator
 	$ctxm add command -label [mc "Options..."] \
 		-command do_options
 }