subrepo: try remapping subpaths using the "final" path
Before, the right-hand side of a .hgsub entry was used, as is, to
match the left-hand side of a subpaths entry. This turned out to be
less useful than expected since a .hgsub file with
src/foo = src/foo
has little context to do remapping on. The new idea is therefore to
prefix the parent repo path *before* the remapping takes place.
If the parent repository path (as defined by _abssource) is
http://example.net/parent
then the remapping for the above .hgsub entry will be done on the
expanded path:
http://example.net/parent/src/foo
If this expanded path is not changed by the remapping, then we remap
src/foo
alone. This is the old behavior where the right-hand side is remapped
without context.