scripts/i18n: add command 'normalized-merge' for use with Mercurial's 'merge-tool' option

Add a 'normalized-merge' command to scripts/i18n that will first normalize
the i18n files contributing to the merge, then perform a standard merge. If
that merge fails (e.g. due to real conflicts) the normalized files are left
behind, and the user needs to run another merge tool manually and resolve the
merge of these.

Use by putting following snippets in your .hgrc file:

[merge-tools]
i18n.executable = /path/to/scripts/i18n
i18n.args = normalized-merge $local $base $other $output

or

[merge-tools]
i18n.executable = python3
i18n.args = /path/to/scripts/i18n normalized-merge $local $base $other $output

and when i18n files conflict, get all 3 sides of the merge normalized before
merge by running:

hg resolve 'kallithea/i18n/*/LC_MESSAGES/kallithea.po' --tool i18n