One pretty major change between the way post-review handled configs and the way

rbt handled configs is that in post-review, $HOME/.reviewboardrc was considered

last, whereas in rbt, it was considered first. I've changed our new

load_config implementation so that any .reviewboardrc files in your current

directory (or a parent of your current directory) take priority over the one in

the home directory.
As I did this, I noticed that the way config was passed around was pretty

crazy. Once upon a time the clients had a list of each config file, and then a

separate entry for the user_config from the home directory. After our big

clients/commands refactors, the configs list had only one item, and it was

the same as the user_config. We also still had the old load_config_files

implementation, which was still used in some unit tests.
I've cleaned all this up so that we instantiate SCMClients with a single config

dictionary, which is populated according to the above search order. There's no

more separate configs vs. user_config, and everything is nice and clean.
Testing done:

- Ran unit tests.

- Ran rbt list-repo-types (for the extra load_scmtools() call)

- Posted this change.

Ran unit tests.
Ran rbt list-repo-types (for the extra load_scmtools() call)
Posted this change.