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.
% Repository data:
Some stuff to ponder very carefully.
% Fresh checkout:
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
Copyright 2046, The Masters
Some stuff to ponder very carefully.
M stuff.txt
% Problem encoding:
abort: missing header "Copyright 2046, The Masters" in morestuff.txt
M stuff.txt
A morestuff.txt