Issue 2949:  Refactor upload.py in preparation for other VCS implementations. * Add GuessVCS() that examines the working directory and returns a VersionControlSystem instance. * Call GuessBase() only when used with Subversion.
Issue 2949:  Refactor upload.py in preparation for other VCS implementations.
* Add GuessVCS() that examines the working directory and returns a VersionControlSystem instance.
* Call GuessBase() only when used with Subversion.

git-svn-id: 095a516e186c72f46b708383d0d3f28c2432e8a1@252 e30e9e8f-1b4c-0410-881d-6f13584adfe0

imirkin/rietveld,termie/rietveld-mirror,imirkin/rietveld,termie/rietveld-mirror