Improve syncing and saving behavior

[maxg]

1. Partner edits to a file you do not have open are not propagated until you open the file and confirm the merge.
2. Partner edits to a file you have open but are not currently editing are not saved until you switch to that file and save it. This e.g. creates confusing compilation errors because you must save changes you didn't write to trigger recompilation.

Possible approach is to treat save as a shared document operation. For (2), when your partner saves the file, that triggers a local save.

For (1), we might:

• Treat open as a shared operation, so that partner edits are always on open files.
• Check whether saving an unopened file is "safe" by comparing its contents to the last save (or version 0). The same check could determine whether opening a file and applying unsaved partner edits is safe, too, avoiding the merge dialog.