Fix the check for updated msgids
When updating the msgids in an existing po translation, the diff from the existing .po file to the the update-po-from-pot.pl output was only checking lines starting with "[+-]msg", in order to avoid pushing the file every time there was a change to the .po header. This needs to be changed because diffs in help files (and probably to the second and later lines of multi-line translations) don't start with 'msgid'. Instead, attempt to find diffs on lines following the "X-Generator: Launchpad" or "X-Launchpad-Export-Date:" lines (hopefully in the header).
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