R1:ed5c46681dc3

Allow SVN repositories to import subdirectories instead of the entire repository

Summary:
See T325. While this is a touch hacky it ends up being fairly clean, and we can
now do initial imports much more quickly and this actually cleaned up some of
the code. I also made the repository edit interface a little less foreboding.

@tuomaspelkonen, did you get anywhere with that bug you were chasing down a
couple days ago? We can hold this if it throws a wrench into stuff you're
working on.

Test Plan…