[merge] fix up and enforce style of option help

[Martin Pool]

(not for 0.18)

At the moment we have the minor but noticeable ugliness that different
options to the same command have different punctuation, viz:

Options:
  -p ARG, --prefix=ARG  Set prefixes to added to old and new
filenames, as two values separated by a colon. (eg "old/:new/")
  --diff-options=ARG
  -h, --help            show help message
  -r ARG, --revision=ARG
                        See 'help revisionspec' for details

This patch

 * Adds text to HACKING explaining that they should be in sentence
case with a trailing period.  This is arbitrary but reasonable, I
think, and scales better to help messages that need more than one
sentence.

 * Adds a test that all options defined globally or in commands
visible when the test suite is run comply with this.

 * Fixes the ones that don't.

 * Incidentally improves the phrasing of some messages and formatting
of some code.

-- 
Martin
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