Verify that SVN repository URI ends in a "/"

Summary:
Everything breaks if this isn't true, and it's easy to get subtly wrong right
now. There are other more magical ways we could do this (automatically add a "/"
in this form or at runtime) but I think making it explicit is the easiest and
most robust approach. See T67.

Test Plan:
Tried to save a URI without a trailing slash.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 305