Add fields to vcp:
source_name,
source_filebranch_id,
source_branch_id,
source_rev_id,
source_change_id
1. Alter revml.dtd to include the fields
2. Alter bin/gentrevml to emit legal RevML
3. Extend VCP::Rev to have the fields
4. Extend VCP::{Source,Dest}::revml to read/write the fields
(VCP::Dest::revml should die() if VCP tries to emit illegal
RevML)
5. Extend VCP::{Source,Dest}::{cvs,p4} to read the fields
7. Get all tests through t/91*.t to pass
except those that rely on ch_4 labels