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.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.