gitk: commit translation files without file information

gitk: commit translation files without file information

File information in the translation files is only helpful for the
translators, but is not needed to compile the message catalogs. On top
of that, file information is rather volatile and leads to large patches
that do not carry essential information. For this reason, Git project
has opted to remove the file information from its translation files.
Let's do that in this project, too.

Rewrite the update-po target to generate *.po files that do contain
file information for the benefit of translators. Configure a clean
filter under the name "gettext-no-location", which is the same that
the Git project uses. It is expected that translators have already
configured their repository suitably. Nevertheless, write a reminder
as part of the update-po target.

Apply the clean-filter to the translation files.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>