Moved VCS detection from _vcs_help() to _vcs_version().

Moved VCS detection from _vcs_help() to _vcs_version().

The version string is useful information to have around, while the
help string is probably not.  For example, we use it in
darcs.Darcs._vcs_get_file_contents() to construct an incantation
appropriate to the version we're dealing with.