Saving is much faster on large graphs: a save no longer re-reads and re-analyses
every page before writing.

"Keep mine" now works when a page changed on disk behind Tine's back. It could
never succeed before, leaving discarding your own edits as the only way out.

Fixes for saves that could get stuck for good: a file replaced with identical
content, a symlink in the graph, and errors wrongly shown as "changed on disk".

Edits made outside Tine now appear for pages kept anywhere in your graph.
