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Optional explicit edit command
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The default is for each branch to have an editable working copy, which
can be modified and committed.  However, in some cases (following RCS
and BK) people might prefer to have either a read-only working copy,
or none at all.

Reasons:

 - Branches which are never edited don't need it; removing it saves
   some time and space.  In particular this applies to branches on a
   server which are meant to be branched from or committed too, not
   edited.

 - Files for which there is no working copy or a read-only copy are
   known not to be edited, and this can make some operations such as
   diffing a bit simpler.

Since this is mostly an optimization and requires user knowledge it
perhaps should not be added.  Certainly not yet.

Sketch::

  $ bzr get --no-wc SOURCE TARGET
  $ bzr edit --remove
  $ bzr edit --read-only .
  $ bzr edit --write foo.c