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

mercurial/metadata.py

@@ -1,4 +1,4 b''
-# coding: utf8
+# coding: utf-8
 # metadata.py -- code related to various metadata computation and access.
 #
 # Copyright 2019 Google, Inc <[e-mail]>