git-gui: display summary when showing diff of a submodule

As it is hard to say what changed in a submodule by looking at the hashes,
let's show the colored submodule summary instead.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -3212,7 +3212,7 @@ proc popup_diff_menu {ctxm ctxmmg x y X Y} {
 			set l [mc "Stage Hunk For Commit"]
 			set t [mc "Stage Line For Commit"]
 		}
-		if {$::is_3way_diff
+		if {$::is_3way_diff || $::is_submodule_diff
 			|| $current_diff_path eq {}
 			|| {__} eq $state
 			|| {_O} eq $state

lib/diff.tcl

@@ -255,7 +255,7 @@ proc show_other_diff {path w m cont_info} {
 
 proc start_show_diff {cont_info {add_opts {}}} {
 	global file_states file_lists
-	global is_3way_diff diff_active repo_config
+	global is_3way_diff is_submodule_diff diff_active repo_config
 	global ui_diff ui_index ui_workdir
 	global current_diff_path current_diff_side current_diff_header
 
@@ -265,6 +265,7 @@ proc start_show_diff {cont_info {add_opts {}}} {
 	set s $file_states($path)
 	set m [lindex $s 0]
 	set is_3way_diff 0
+	set is_submodule_diff 0
 	set diff_active 1
 	set current_diff_header {}
 
@@ -295,6 +296,11 @@ proc start_show_diff {cont_info {add_opts {}}} {
 		lappend cmd $path
 	}
 
+	if {[string match {160000 *} [lindex $s 2]]
+        || [string match {160000 *} [lindex $s 3]]} {
+		set cmd {submodule summary -- $current_diff_path}
+	}
+
 	if {[catch {set fd [eval git_read --nice $cmd]} err]} {
 		set diff_active 0
 		unlock_index
@@ -312,7 +318,7 @@ proc start_show_diff {cont_info {add_opts {}}} {
 }
 
 proc read_diff {fd cont_info} {
-	global ui_diff diff_active
+	global ui_diff diff_active is_submodule_diff
 	global is_3way_diff is_conflict_diff current_diff_header
 	global current_diff_queue
 	global diff_empty_count
@@ -337,6 +343,9 @@ proc read_diff {fd cont_info} {
 		}
 		set ::current_diff_inheader 0
 
+		if {[regexp {^\* } $line]} {
+			set is_submodule_diff 1
+		}
 		# -- Automatically detect if this is a 3 way diff.
 		#
 		if {[string match {@@@ *} $line]} {set is_3way_diff 1}
@@ -374,6 +383,23 @@ proc read_diff {fd cont_info} {
 				set tags {}
 			}
 			}
+		} elseif {$is_submodule_diff} {
+			if {$line == ""} continue
+			if {[regexp {^\* } $line]} {
+				set line [string replace $line 0 1 {Submodule }]
+				set tags d_@
+			} else {
+				set op [string range $line 0 2]
+				switch -- $op {
+				{  <} {set tags d_-}
+				{  >} {set tags d_+}
+				{  W} {set tags {}}
+				default {
+					puts "error: Unhandled submodule diff marker: {$op}"
+					set tags {}
+				}
+				}
+			}
 		} else {
 			set op [string index $line 0]
 			switch -- $op {