tests/test-dirstate-backup.t

url: move _wraphttpresponse() from httpeer

This is a generally useful function. Having it on the url module
will make it more accessible outside of the HTTP peers.

Differential Revision: https://phab.mercurial-scm.org/D4770

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