dgit: Transfer origs from .. to bpd when fetching

dgit: Transfer origs from .. to bpd when fetching

This is the second half of #904878.

When fetching we want to avoid downloading once again origs that are
already in `..', but not in bpd.  So call the facility we now have for
making clones of things from `..' to bpd.

`..' might contain various junk.  We try to treat it with a bit of
suspicion.  In particular, when we have a dsc we know exactly which
orig files we are looking for.

So only try to link those.  We don't do this transfer for debian
tarballs or diffs, or for origs that would be related according to
their filenames, but aren't in the dsc.

This maximises the probability that fetch will succeed even if there
is garbage in the `..'.  (Garbage in the `..' will sometimes
unavoidably cause source builds to fail, because when doing a source
build we don't know what origs to expect.)

Closes: #904878
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>