We must not clobber the original message string, as Email::MIME(*)
still needs it for iterating through parts in SearchIdx (but not
when handing it as a raw string to git-fast-import).
I've noticed message bodies (especially dfpre/dpost) were not
getting indexed when going through -mda (no problems with
-watch). This also did not affect v1 repos, since indexing is a
separate process for v1 and requires re-reading the data from
git.
(*) tested Email::MIME 1.937 on Debian stretch