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.

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.