Limit multi-commit review requests to repositories that support them

Review Request #9883 — Created April 20, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
69fdb9d...

Reviewers

SCMTools now have a supports_history flag that indicates whether or
not they support multi-commit review requests. The API now prevents
creating a review request with history when the underlying SCMTool does
not support it.

For now, only Git will have the support enabled (althrough incomplete)
until the overall DVCS support is feature complete and then support for
other SCMs can be added.

Ran unit tests.