Set Content-Transfer-Encoding to 8bit.

Set the Content-Transfer-Encoding so MTAs know what kind of mangling
might be needed when sending to a non 8-bit clean SMTP host.

This commit is derived from a patch by Jonathan Nieder to
post-receive-email, submitted to the Git mailing list.

Adapted for git-multimail by: Michael Haggerty <mhagger@alum.mit.edu>
Define and use named constants ENCODING and CHARSET.
Set i18n.logoutputencoding when invoking git commands.

Explicitly ask git to output commit messages in UTF-8.
Merge branch 'encoding'

Work a little bit harder to generate get clean UTF-8 emails in the
presence of non-ASCII characters.