It replaces the msgstr (en_GB) by the content of the msgid (en_US).
Therefore if one does a diff afterwards, one can easily identify the
messages for which the two languages differ. One can then quickly
check if the current en_GB message (the - lines) is still correct
w.r.t. the original en_US one (the + lines).
Usage:
{{{
python contrib/l10n_reset_en_GB.py trac/locale/en_GB/LC_MESSAGES/*.po
}}}
then examine the diff.
This can help to detect errors like introduced due to fuzzy matching,
and translations not fully updated, but of course this won't help to
detect the messages which are identical but //should// have been
modified.
