fast-import: support 'encoding' commit header

fast-import: support 'encoding' commit header

Since git supports commit messages with an encoding other than UTF-8,
allow fast-import to import such commits.  This may be useful for folks
who do not want to reencode commit messages from an external system, and
may also be useful to achieve reversible history rewrites (e.g. sha1sum
<-> sha256sum transitions or subtree work) with git repositories that
have used specialized encodings in their commit history.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>