export: write a message once, in every folder it was in

A source that files one message in several folders -- IMAP and Maildir
copies, Gmail labels -- leaves one archive row per folder, all pointing at the
same blob. Export imported each row on its own, so the message arrived on the
target once per folder. Worse, an interrupted export matched the rest by
Message-ID alone on the next run and skipped them, so their folders were never
added.

Export now folds rows with the same blob into one email, with the union of
their folders and keywords. On a match it adds, with one batched Email/set,
any migrated folder the target copy is missing; folders that exist only on the
target are left alone. Matching is still by Message-ID (or the fallback
digest), with size deciding between several messages that share one, so two
different messages are never folded onto one target email.

The archive is unchanged: the fold happens at export, so imports, resumes and
deletions work exactly as before.

docs/usage.md

@@ -196,6 +196,12 @@ By default it only adds and updates -- items that match are updated, the
 rest are created, and anything already on the target that the archive does
 not cover is left alone.
 
+Email is matched by Message-ID, or without one by sender, subject, date and
+recipients; where several messages share one, size decides. A message the
+source kept in several folders -- IMAP and Maildir copies, Gmail labels -- is
+written once, in all of them, and a later run adds any folder it is still
+missing on the target.
+
 `--prune` also deletes what is on the target and not in the archive. It asks
 first; `--yes` answers for it, for scripts. Export speaks JMAP only.