Without the presence of a .reviewboardrc file which defines

REPOSITORY_TYPE, RBTools will try to auto-detect the repository type by running

various commands like "svn info", "hg root", etc. This will run through each

repository type until one is found that works. 
Unfortunately, if people have nested repositories, this means

RBTools can select the wrong one. For example, some people may have

/home/user/ as a git directory, and /home/user/src/project/ is a subversion

checkout, running "rbt" within their "project" directory may detect it as git

instead of svn. 
This change auto-detects all repository types, and then selects the

deepest one.  In the case of multiple matching repositories, we output a

warning suggesting that people define REPOSITORY_TYPE in .reviewboardrc in

order to speed up detection.

Tested combinations of nested svn, git, hg repos.
All unit tests pass.