Git commit format changes

[Guillem Jover <guillem@debian.org>]

Hi!

I've prepared a script to help with generating debian/changelog from
the git log output, but that requires for commit messages to follow
a specific format. For translators, in my next push, this will be
documented in the README.l10n file (which I'm also attaching), I'll
probably merge some doc for the general commit format instead of
relying on the wiki.

Just a heads-up that debian/changelog should not be modified any
longer and that the commit messages need to follow the new format. :)

Thanks,
Guillem

Translators, when adding/updating your translation files, please follow
the following rules:

* Do not update debian/changelog:

  - The file will get generated out of the commit messages at release time.
  - Beware that you should NOT update any of the legacy ChangeLog.old
    files for translation updates.

* Format of commit message

  Following guidelines in <https://wiki.debian.org/Teams/Dpkg/GitUsage>
  you should start the commit message with a summary line, followed by an
  empty line and optional pseudo-headers.

  The summary line should use the following format:

  «po: <Action> <Language> <part> translation»

  Where:

  - <Action> is one of “Update” or “Add”,
  - <Language> should be capitalized (such as “English”),
  - <part> should be one of «programs» (for lib, src/, dpkg-deb/, dpkg-split/
    and utils/), «dselect», «scripts» and «man pages» (man/).

  For example:

==========================================================
po: Update German programs translation

Closes: #123456
Reviewed-by: Full Name <email@example.org>
==========================================================

* Use of po/LINGUAS, dselect/po/LINGUAS or scripts/po/LINGUAS:

  When ADDING a new translation, don't forget adding the language to
  the LINGUAS file, otherwise it will not be used.

* Always CHECK your translations:

  You MUST check your PO files for validity.

  The correct syntax for doing so is:

  $ msgmerge -U <file> dpkg.pot
  $ msgfmt -c -o /dev/null --statistics <file>
  $ msgcat <file> >/dev/null

  - msgmerge updates your file with the current POT file.
  - msgfmt checks it for validity.
  - msgcat may detect encoding problems.

  In addition to the above, the easiest way to check that the po4a
  translations have valid POD markup, is to try to build them (please
  see the Building sections in the README file for further details
  regarding the setup of the source tree):

  $ make -C man