fast-export: do automatic reencoding of commit messages only if requested

fast-export: do automatic reencoding of commit messages only if requested

Automatic re-encoding of commit messages (and dropping of the encoding
header) hurts attempts to do reversible history rewrites (e.g. sha1sum
<-> sha256sum transitions, some subtree rewrites), and seems
inconsistent with the general principle followed elsewhere in
fast-export of requiring explicit user requests to modify the output
(e.g. --signed-tags=strip, --tag-of-filtered-object=rewrite).  Add a
--reencode flag that the user can use to specify, and like other
fast-export flags, default it to 'abort'.

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