pretty: get the correct encoding for --pretty:format=%e

pretty: get the correct encoding for --pretty:format=%e

parse_commit_header() provides the commit encoding for '%e' and it
reads it from the re-encoded message, which contains the new encoding,
not the original one in the commit object. This never happens because
--pretty=format:xxx never respects i18n.logoutputencoding. But that's
a different story.

Get the commit encoding from logmsg_reencode() instead.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>