CMake: Clean up VersionFromVCS.cmake

Summary:
Fix a few problems in VersionFromVCS.cmake to make it more reliable:

- Stop using git svn info to retrieve the svn revision.  I am unable to
  determine what the svn revision returned by this command means.
  During my testing this command returned a revision from a month
  ago which was not the HEAD of any of my local branches.

  Also, this revision was never actually added to the version string due
  to a typo in the script.  All it was used for was to reject the
  revision number returned by git svn find-rev HEAD when the revision
  numbers didn't match.

- Populate GIT_COMMIT even when we detect a git repo without any
  svn information.

Reviewers: mehdi_amini, beanz

Reviewed By: beanz

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D30092

llvm-svn: 296829