The default behaviour for posting is now to determine the capabilities

of the server to see if it supports creating a review request with

commit history. The user can specify the -S/--squash-history

option or SQUASH_HISTORY config option in .reviewboardrc to

override this. However, the configuration option can itself be

overridden by the -H/--with-history flag.
Currently the only SCMClients that "supports" this flag is Git.
Add the DiffCommitResource and DiffCommitListResource resources to

the RBTools web API resources. The DiffCommitListResource handles

the uploading of commits via the upload_commit method.
Add the create_empty_diffset method to the DiffListResource class.

This method is used to create an empty DiffSet so that a review

request can be created with commit history.
Add the get_history method to the SCMClient class. This method

gets a linear history of commit metadata given some revisions.
The commit histories that can be uploaded by this change are simple

linear ones; that is, they must be linear histories (with no merges)

and diffs in commits should not overlap with respect to files.

Uploaded commits containing diffs to Review Board on the dvcs

branch.