i18n: fix coding tag unsupported by xgettext
Running `make update-pot` currently fails with the following error:
xgettext: mercurial/metadata.py:1: Unknown encoding "utf8". Proceeding with ASCII instead.
xgettext: Non-ASCII string at mercurial/metadata.py:311.
Please specify the source encoding through --from-code or through a comment
as specified in http://www.python.org/peps/pep-0263.html.
Differential Revision: https://phab.mercurial-scm.org/D9260
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).