obsolete: store user name and note in UTF-8 (issue5754) (BC)
Before, user names were stored in local encoding and transferred across
repositories, which made it impossible to restore non-ASCII user names on
different platforms. This patch fixes new markers to be encoded in UTF-8
and decoded back to local encoding when displaying. Existing markers are
unfixable so they may result in mojibake.
I don't like the API that requires metadata dict to be UTF-8 encoded, which
is a source of bugs, but there's no abstraction layer to process the encoding
thingy efficiently. So we apply the same rule as extras dict to obsstore
metadata.