dgit: New fetch algorithm - try to tolerate in-archive copies

dgit: New fetch algorithm - try to tolerate in-archive copies

Big comment explains.

Amongst other things:
 * Introduce the mergeinfo concept
 * Abolish fetchspec in favour of lrfetchref
 * Synthesize a pseudo-merge linking the archive's .dsc with the dgit
   server's git branch, as appropriate.
 * Introduce mergeinfo_version.

I have tried to ensure that where the import done by previous versions
of dgit would be correct, the new import is the same.

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