Makefile: wrap hg.pot strings at 82 chars
By default, xgettext will wrap strings at 79 characters. This width
includes the two double-quotes and the '\n' added. Our strings are now
wrapped at 78 characters and a line can therefore be up to 82
characters wide in the .po file.
Setting the width to 82 characters ensures that xgettext does no silly
wrapping on our already-wrapped strings while still wrapping long
one-liners at 82 characters.