fetch: add a failing test for prunning with overlapping refspecs

When a remote has multiple fetch refspecs and these overlap in the
target namespace, fetch may prune a remote-tracking branch which still
exists in the remote. The test uses a popular form of this, by putting
pull requests as stored in a popular hosting platform alongside "real"
remote-tracking branches.

The fetch command makes a decision of whether to prune based
on the first matching refspec, which in this case is insufficient, as it
covers the pull request names. This pair of refspecs does work as
expected if the more "specific" refspec is the first in the list.

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This setup is used by GitHub for Windows, but we haven't seen this break
because it puts the PR refspec in the system config, which makes that
one the first. I was alerted to this by someone who had done this
setup manually and thus added the PR refspec after the default one.
fetch: handle overlaping refspecs on --prune

We need to consider that a remote-tracking branch may match more than
one rhs of a fetch refspec. In such a case, it is not enough to stop at
the first match but look at all of the matches in order to determine
whether a head is stale.

To this goal, introduce a variant of query_refspecs which returns all of
the matching refspecs and loop over those answers to check for
staleness.