tests/test-dirstate-backup.t

narrow: check for servers' narrow support before doing anything (BC)

Recently we introduced narrow capabilities for the server. So we can check
whether a server has narrow clone support or not before doing anything. This is
BC because new clients won't be able to extend from old narrow-enabled servers.
I *think* narrow is not used much (maybe just inside Google), also it's
experimental so I think we can change this. We will need to this someday anyway.

The "doesn't" in error is changed to "does not" because I think that's we do in
core. I also changed one more instance of the error message to use 'does not'
for consistency.

Differential Revision: https://phab.mercurial-scm.org/D4789

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