Add Diff to contextual menus

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

PBDiffWindowController.m

@@ -7,6 +7,8 @@
 //
 
 #import "PBDiffWindowController.h"
+#import "PBGitRepository.h"
+#import "PBGitCommit.h"
 
 
 @implementation PBDiffWindowController
@@ -21,4 +23,32 @@
 	return self;
 }
 
+
++ (void) showDiffWindowWithFiles:(NSArray *)filePaths fromCommit:(PBGitCommit *)startCommit diffCommit:(PBGitCommit *)diffCommit
+{
+	if (!startCommit)
+		return;
+
+	if (!diffCommit)
+		diffCommit = [startCommit.repository headCommit];
+
+	NSString *commitSelector = [NSString stringWithFormat:@"%@..%@", [startCommit realSha], [diffCommit realSha]];
+	NSMutableArray *arguments = [NSMutableArray arrayWithObjects:@"diff", commitSelector, nil];
+	if (filePaths) {
+		[arguments addObject:@"--"];
+		[arguments addObjectsFromArray:filePaths];
+	}
+
+	int retValue;
+	NSString *diff = [startCommit.repository outputInWorkdirForArguments:arguments retValue:&retValue];
+	if (retValue) {
+		NSLog(@"diff failed with retValue: %d   for command: '%@'    output: '%@'", retValue, [arguments componentsJoinedByString:@" "], diff);
+		return;
+	}
+
+	PBDiffWindowController *diffController = [[PBDiffWindowController alloc] initWithDiff:[diff copy]];
+	[diffController showWindow:nil];
+}
+
+
 @end