Honour the fetch ref-spec in replication.config

When an individual ref was fetched as a result of invoking
the fetch REST-API, the ref-spec built on-the-fly did not
respect the ref-specs configured in replication.config.

Even worse, refs that were NOT supposed to be fetched were
not discarded, causing potentially unwanted replications.

Bug: Issue 15426
Change-Id: I7ca9f9de39ec914125b0389dc2ae35054ccd80af