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.

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