changelog: refuse to add revisions with empty usernames
An empty username or a username with a "\n" will make the revision
text contain two "\n\n" sequences -> corrupt repository.
The problem is that changelog.read expects to find exactly one "\n\n"
separator and thus cannot unpack the revision.
% try to commit on top of a patch
abort: cannot commit over an applied mq patch
% qpop/qrefresh on the wrong revision
abort: popping would remove a revision not managed by this patch queue
using patch queue:
abort: popping would remove a revision not managed by this patch queue
abort: working directory revision is not qtip
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% qpop
abort: popping would remove a revision not managed by this patch queue
% qrefresh
abort: cannot refresh a revision with children
% tip:
3 append quux