revisions: a save keeps what it replaces

Every write of a document's text keeps the text it replaces, in the same
transaction. Autosave writes every few seconds, so an ordinary save keeps
one at most every ten minutes; `:w!`, a save over the SFTP mount and a
restore always keep one, because each may undo something. A rename alone,
or a document that was empty, keeps nothing.

`gh` or `:history` lists the open document's earlier versions, the newest
first, and shows the highlighted one as a line diff against the text as it
is now. Enter brings it back, and what it replaces becomes a version of
its own, so a restore is undone the same way.

The daily sweep forgets versions older than WORKSPACE_REVISION_DAYS (90)
but never the newest 20 of a document, and keeps an upload as long as a
version still embeds it. Versions stay out of the export, the mount and
the readable mirror; a document in the trash shows none.

web/src/lib/Help.svelte

@@ -43,6 +43,7 @@
     ['y', 'mint a new share link and copy it — asks, the old one stops working'],
     ['x · dd', 'delete what has focus — the row in the list, the document in its pane — into the trash (asks y/N)'],
     ['u', 'undo the last delete: it comes back out of the trash — :trash has the rest'],
+    ['gh', "the open document's earlier versions, as diffs — Enter brings one back"],
     [':', 'command line — Tab / Shift-Tab cycle the matches, then the argument; ↑ ↓ recall what ran before'],
     ['Ctrl-S', 'save, from anywhere'],
     ['?', 'this screen — in the editor too'],