Add the -H and --with-history options to the post command, which

allow creation of a multi-commit review request if the repository's

SCMClient supports it.
Currently no SCMClients support the -H flag.
Add the SCMClient.get_revision_history method which calculates the 

revision history of the provided revision dictionary. It produces a

list of revision dictionaries that can be used to create diffs for

each commit in the range of commits.
Add an implementation of get_revision_history for the Git SCMClient.

Running rbt post -H ... with any SCMClient fails.
Unit tests pass.