Don't keep fetching the same graphite-web revision

The vcsrepo module doesn't really like the combo of latest and a
specific sha. It keeps grabbing it over and over again. Guess what?
The sha hasn't updated since the last time you looked for the same
sha.

Change-Id: I1a017278b83af11541794bf03644b3346e9869c2
Merge "Don't keep fetching the same graphite-web revision"