Enforce prefixes "a/" and "b/" when doing git diff

The user can set different prefix using diff.mnemonicprefix or
diff.noprefix (see git-config(1)), so we need to force them to the
values that the diffeditor and PatchTool expect.

Task-number: QTCREATORBUG-13782
Change-Id: I3110a34b2575b89abe840e040690f3f0c36e955b
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gitclient.cpp

@@ -270,6 +270,7 @@ void GitDiffHandler::addJob(VcsCommand *command,
         args << QLatin1String("--ignore-space-change");
     args << QLatin1String("--unified=") + QString::number(
                 m_controller->contextLinesNumber());
+    args << QLatin1String("--src-prefix=a/") << QLatin1String("--dst-prefix=b/");
     args << arguments;
     command->addJob(args, timeout());
 }