Avoid processing internal Gerrit refs

When Gerrit internal refs are updated, the its-base plugin should
not consider them as actionable because they do not necessarily
point to any valid Git commit object.

refs/starred-changes/ and refs/sequences/ are internal and thus
they should always be ignored.

Bug: Issue 11097
Change-Id: Ic5099d67b7bd7e757de8305c4a60ddbbbebf1b0d