gerrit-git-prep would fail if a single git fetch was unable to get
what it requested from Gerrit. These errors occur due to server side
issues and gerrit-git-prep should simply try again. This patch has
gerrit-git-prep attempt each git fetch at least three times with a
random sleep interval between 30 and 90 seconds between each fetch.
Change-Id: Ib7f9292ec6e55ee7e6d3a5fe832c302660a1aa00
f7e11a621d