Make the --username option unnecessary for the DVCS outputs.
The DVCSs need a username for synthesized commits, so set a default
rather than making it mandatory for the user to select one. As
default, use the name of the conversion tool ("cvs2git", "cvs2bzr", or
"cvs2hg").
Leave the value unset by default for cvs2svn, because Subversion
allows commits without an explicit author.
Suggested by: Eric S. Raymond <esr@thyrsus.com>
git-svn-id: http://cvs2svn.tigris.org/svn/cvs2svn/trunk@5414 be7e6eca-30d4-0310-a8e5-ac0d63af7087