p4_dscm. Update todo list
p4_dscm. Remove the checkout master head in command revert to master.

The call to checkout the HEAD of master when reverting to master is not
needed, since there shouldn't be any modification on the git repo.
This just slowed down performance of every command for large repositories.
p4_dscm. Add discard command.

Add the discard command to drop changes that will not be submitted.
Merge branch 'p4_dscm'