Don't get VCS version in VCS.__init__().

Don't get VCS version in VCS.__init__().

Often, this just causes a slow subprocess.Popen() call to
get information we woln't even look at.

Old benchmark:
  $ time be list > /dev/null

  real    0m2.369s
  user    0m1.980s
  sys     0m0.388s
New benchmark:
  $ time be list > /dev/null

  real    0m1.472s
  user    0m1.304s
  sys     0m0.164s