option help text grammar is inconsistent

  affects bazaar

> [3] mbp@kapu% ./bzr tags -h
> usage: bzr tags
>
> List tags.
>
> This tag shows a table of tag names and the revisions they reference.
>
> Options:
> -d ARG, --directory=ARG
> Branch whose tags should be displayed
> -h, --help show help message

Some options help messages have sentence case and some are all in lower
case. Some have trailing periods and some do not.

We should perhaps add a test that they're all ok, and fix up those that
are not.

I suggest that they should all have an initial capital, but (as they are
not proper sentences) not have a trailing period.

--
Martin