Add Diff to contextual menus

- added to menus for refs, commits, and files
    - show a message when there are no changes

PBRefController.m

@@ -12,6 +12,7 @@
 #import "PBCreateBranchSheet.h"
 #import "PBCreateTagSheet.h"
 #import "PBGitDefaults.h"
+#import "PBDiffWindowController.h"
 
 @implementation PBRefController
 
@@ -204,6 +205,19 @@
 }
 
 
+#pragma mark Diff
+
+- (void) diffWithHEAD:(PBRefMenuItem *)sender
+{
+	PBGitCommit *commit = nil;
+	if ([[sender refish] refishType] == kGitXCommitType)
+		commit = (PBGitCommit *)[sender refish];
+	else
+		commit = [historyController.repository commitForRef:[sender refish]];
+
+	[PBDiffWindowController showDiffWindowWithFiles:nil fromCommit:commit diffCommit:nil];
+}
+
 #pragma mark Tags
 
 - (void) createTag:(PBRefMenuItem *)sender