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:

• Imported a subdirectory of a midsized SVN project (jQuery UI).
• Commit discovery for ~3500/4500 commits took just a few seconds.
• Commit discovery correctly ignored commits which didn't affect this

directory.

• Commit discovery correctly stopped at commit 13.
• Browse interface shows an incomplete listing, but that's fine, and

everything is otherwise functionally correct. We can add a note or something
later ("this is a view of commits affecting a subdirectory, some paths aren't
available"), but this behavior probably won't be too startling to users.

• Edited Git and SVN repositories to test form logic.

Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran, Girish
Commenters: tuomaspelkonen
CC: jcleveley, aran, jungejason, tuomaspelkonen
Differential Revision: 696