match: add prefixdirmatcher to adapt subrepo matcher back...
match: add prefixdirmatcher to adapt subrepo matcher back
This serves as an inverse function to the subdirmatcher, and will be used
to wrap a fileset matcher of subrepositories. One of the root/prefix paths
could be deduced from the matcher attributes to be wrapped, but we don't
since the callers of this class know the root/prefix paths and can simply
pass them in.