tests/test-dirstate-backup.t

lfs: infer the blob store URL from an explicit pull source

I don't see any easier way to do this because the update part of `hg pull -u`
happens outside exchange.pull(), and commands.postincoming() doesn't take a
path.  So (ab)use the mechanism used by subrepos to redirect where subrepos are
pulled from when an explicit path is given.  As a bonus, this should allow lfs
blobs to be pulled into a subrepo when it is checked out.

An explicit push path can be handled within exchange.push().  That can be done
next, outside of this dirty hack.

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [255]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate