Allow opening new changes on existing commits

The %base argument can be used with refs/for/ to identify a specific
revision the server should start to look for new commits at. Any
commits in the range $base..$tip will be opened as a new change,
even if the commit already has another change on a different branch.

Change-Id: Ic2ea9b7c53df3b99d29043cd6a6fd68214f3398f
Merge "Allow opening new changes on existing commits"