VcsCommand: Rename execute() to start()

In order to conform to QtcProcess API.

Change-Id: I71fcc03846f32cd3c2df39824ce3f6730c20ebbb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/wizard/vcscommandpage.cpp

@@ -374,7 +374,7 @@ void VcsCommandPage::start(VcsCommand *command)
     m_statusLabel->setText(m_startedStatus);
     m_statusLabel->setPalette(QPalette());
     m_state = Running;
-    command->execute();
+    command->start();
 
     wizard()->button(QWizard::BackButton)->setEnabled(false);
 }