Adds CodeLens for Diff'ing in blame

Other fixes and refactoring

src/commands.ts

@@ -2,6 +2,7 @@
 import {commands, Disposable, Position, Range, Uri, window} from 'vscode';
 import {Commands, VsCodeCommands} from './constants';
 import GitProvider from './gitProvider';
+import {basename} from 'path';
 
 abstract class Command extends Disposable {
     private _subscriptions: Disposable;
@@ -41,4 +42,33 @@ export class BlameCommand extends Command {
             return commands.executeCommand(VsCodeCommands.ShowReferences, uri, position, locations);
         });
     }
+}
+
+export class DiffWithPreviousCommand extends Command {
+    constructor(private git: GitProvider) {
+        super(Commands.DiffWithPrevious);
+    }
+
+    execute(uri?: Uri, sha?: string, compareWithSha?: string) {
+        // TODO: Execute these in parallel rather than series
+        return this.git.getVersionedFile(uri.path, sha).then(source => {
+            this.git.getVersionedFile(uri.path, compareWithSha).then(compare => {
+                const fileName = basename(uri.path);
+                return commands.executeCommand(VsCodeCommands.Diff, Uri.file(source), Uri.file(compare), `${fileName} (${sha}) ↔ ${fileName} (${compareWithSha})`);
+            })
+        });
+    }
+}
+
+export class DiffWithWorkingCommand extends Command {
+    constructor(private git: GitProvider) {
+        super(Commands.DiffWithWorking);
+    }
+
+    execute(uri?: Uri, sha?: string) {
+        return this.git.getVersionedFile(uri.path, sha).then(compare => {
+            const fileName = basename(uri.path);
+            return commands.executeCommand(VsCodeCommands.Diff, uri, Uri.file(compare), `${fileName} (index) ↔ ${fileName} (${sha})`);
+        });
+    }
 }