Older versions of RBTools had an undocumented config option called `TREES`, which allowed people to set the `REVIEWBOARD_URL` for multiple remote repository paths from a single .reviewboardrc file in their home directory. After a deprecation cycle, we removed this, but it turns out that some people were using it and had just skipped all the intermediate versions of RBTools and went straight from 1 to 5, missing the deprecation warnings. This change implements a new version of `TREES` which is much more full featured. This allows using either the remote repository path or local directory name as the keys, and the value dictionaries now support the full RBTools configuration. The way this works is that we go through the basic initialization routine to the point where we've initialized the repository. After that, we'll look to see if there's a matching TREES configuration. If there is, we merge that into the base config, remove TREES entirely from the loaded config, and then rerun all initialization. Testing Done: - Set up TREES in my own .reviewboardrc file and verified that things worked as expected. - Added some new tests that cover the three main codepaths for command initialization--no configured server URL, a single server URL, and a server URL that gets overridden inside of TREES. - Ran all unit tests.