VCS: Enable calling runVcs from non-GUI threads

* Introduce a proxy class for thread synchronization
* Use signals for appending text to output window

Change-Id: Iecbb010e6b6e9dab27d9862a43dafa450f2bb1f8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitclient.cpp

@@ -2322,7 +2322,7 @@ Utils::SynchronousProcessResponse GitClient::synchronousGit(const QString &worki
 {
     return VcsBasePlugin::runVcs(workingDirectory, gitBinaryPath(), gitArguments,
                                  settings()->intValue(GitSettings::timeoutKey) * 1000,
-                                 processEnvironment(),
+                                 processEnvironment(), VcsBase::VcsBasePlugin::sshPrompt(),
                                  flags, outputCodec);
 }