Improve performance by re-ordering tests in find_stale_files(). If
we've already found a source file to match the html, exit early -
don't waste time looking for VCS info for it.
Add options to make the output more verbose if desired.
Update boilerplate.

If we're looking at an original language that's not english, don't
look for commit hashes in the english version

If we can't count changes for some reason, report that.

If we can't get details for an original file, report the version as
<UNKNOWN> rather than spurt errors.

Well worth the time taken to improve performance

We need to use "-m" to make sure we see all the changes around
merges, *but* that also gives us noise from the "right-hand" of merges
which can cause confusion - some files will end up shown as being
changed when they actually weren't.

Track each commit hash we look at, and only save the details
the *first* time we see each commit hash. Seems to make all the
difference.

If any of the included files use wml::debian::translation-check with a
version, that over-rides the version defined in the start file and
this can totally break the translation checks. The check is then made
against the start file, but using the version defined in the included
file. :-(

Fix things up to allow for this *and* check the versions of both the
start file *and* the included file:

 * Add a sym-link of translation-check.wml as
   "translation-check-inc.wml" to defeat wml's tracking of what files
   have been #use'd already. Otherwise we'll only check the included
   file and never check the start file.
 * Switch all the .inc files to use the new translation-check-inc.wml
   instead of translation-check.wml
 * Also add an extra argument "force_file" to allow the included files
   to declare their filename so *that* is checked instead of the start
   file

Quite involved, but it works...