Add a get_change method to SCMTool and HostingService

Review Request #4284 — Created July 3, 2013 and submitted — Latest diff uploaded

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master

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Add a get_change method to SCMTool and HostingService

This change adds the final piece of integration API on the backend that we need
for post-commit reviews--the ability to fetch the commit message and diff from
the version control system given a revision ID. This is implemented for SVN and
GitHub, just like the get_branches and get_commits methods.
- Ran unit tests.
- Used this with my post-commit UI for both github and svn repos.