[WIP] Support creation of multi-commit review request with the Git SCMClient

Review Request #6645 — Created Nov. 25, 2014 and discarded — Latest diff uploaded

rbtools, students

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.