git-gui: Allow users to delete branches merged upstream.

Most of the time when you are deleting branches you want to delete
those which have been merged into your upstream source.  Typically
that means it has been merged into the tip commit of some tracking
branch, and the current branch (or any other head) doesn't matter.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1891,7 +1891,7 @@ Delete the above branches?}
 }
 
 proc do_delete_branch {} {
-	global all_heads current_branch
+	global all_heads tracking_branches current_branch
 	global delete_branch_checkhead delete_branch_head
 
 	set delete_branch_checkhead 1
@@ -1932,15 +1932,23 @@ proc do_delete_branch {} {
 	pack $w.list.l -fill both -pady 5 -padx 5
 	pack $w.list -fill both -pady 5 -padx 5
 
+	set all_trackings [list]
+	foreach b [array names tracking_branches] {
+		regsub ^refs/(heads|remotes)/ $b {} b
+		lappend all_trackings $b
+	}
+
 	labelframe $w.validate \
 		-text {Only Delete If} \
 		-font font_ui
 	frame $w.validate.head
 	checkbutton $w.validate.head.r \
-		-text {Already Merged Into Local Branch:} \
+		-text {Already Merged Into:} \
 		-variable delete_branch_checkhead \
 		-font font_ui
-	eval tk_optionMenu $w.validate.head.m delete_branch_head $all_heads
+	eval tk_optionMenu $w.validate.head.m delete_branch_head \
+		$all_heads \
+		[lsort -unique $all_trackings]
 	pack $w.validate.head.r -side left
 	pack $w.validate.head.m -side left
 	pack $w.validate.head -padx 5 -fill x -expand 1