scripts/i18n: introduce --merge-pot-file to control normalization

There are actually *two* kinds of normalization:

- in main branches, where we just want the translations - not any trivially
derived information or temporary or unstructured data.
- in i18n branches, where we want the trivially derived information, and also
want to preserve any other information there might be in the .po files.

If no pot file is specifed, do it as on the main branches and strip everything
but actual translations. This mode will primarily be used when grafting or
rebasing changes from i18n branches.

When a pot file is specified, run GNU msgmerge with it on the po files. The pot
file should ideally be fully updated (as done by extract_messages). That will
establish a common baseline, leaving only the essential changes as needing merge.

If merging from default branches to 18n, it is better to skip .po and .pot in
first 'hg merge' pass, while resolving everything else. Then, with the
uncommitted merge, run 'extract_messages', and then merge the .po files using
--merge-pot-file kallithea/i18n/kallithea.pot .

(Actually, these two different modes could perhaps be auto detected ...)