p4_dscm. Add an option to log output to a log file.
p4_dscm. Start impementation of the change command.

This just includes code changes, it has not been tested, and probably
doesn't work.
p4_dscm. Buf fixes to sync.

Fix full sync so the change list p4 suffix is not added to the git add/rm
commands.
Raise an error if a local path is not a file in p4 depot path to local
path.