digital signatures

[Jonathan Riddell]

> Perhaps we'd need similar support in qlog as well, maybe with similar
> command-line option (or maybe with knob in qlog itself).

I'm just looking at that now.  Probably both added to the output of
qlog and a qverify command

> How does that impact performance? For qlog we'd better put verification into
> separate thread/process?

It does make it slow for large trees (takes 7 minutes on bzr itself),
but one commit is negligable.  So qverify probably needs a thread,
qlog probably not.

Jonathan