git-svn: refuse to dcommit non-UTF-8 messages

git-svn: refuse to dcommit non-UTF-8 messages

...without i18n.commitencoding set in the config.

SVN tries to store all commit messages in UTF-8, however it is
up to the job of the clients to enforce this rule.  SVN servers
themselves do not always enforce this; allowing clients to
commit malformed UTF-8 messages and break repositories.

So git-svn will enforce this and tell the user to set
i18n.commitencoding when a git commit is is not in UTF-8.

Signed-off-by: Eric Wong <normalperson@yhbt.net>