tests/test-empty.out

Tune a bit the extdiff toplevel comments/samples.

* Capitalize the first letter of all sentences

* Add an example for GNU diff(1) 'context diff' mode.

* Explain the requirement for separate cmd.xxx and opts.xxx
  options in .hgrc, which hopefully will guard against users
  trying to add:

      [extdiff]
      # Add a new Mercurial command called `cdiff', which calls
      # GNU diff(1) in 'context diff' mode.
      cmd.cdiff = diff -Nprc5

  which fails for recent crew builds with:

      $ hg cdiff .
      making snapshot of 1 files from rev 07dc4a569f4e
      making snapshot of 1 files from working dir
      diff -Nprc5: not found

  The correct way to do this is by separating the cmd.cdiff and
  opts.cdiff parts like this:

      [extdiff]
      # Add a new Mercurial command called `cdiff', which calls
      # GNU diff(1) in 'context diff' mode.
      cmd.cdiff = diff
      opts.cdiff = -Nprc5

  so add it as a new example and explicitly describe it in the
  extdiff comments.

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