Handle post-commit for SVN repos that don't use the default layout.

When connecting an SVN repo that doesn't use the default layout (trunk/,
branches/, tags/), the "New Review Request" would just show an empty bottom
panel (and have an un-caught exception to boot).

This change makes it so that if we don't see a 'trunk' directory in the root of
the repository, we'll just return the list of directories in the root as the
"branches" instead of nothing.

While I was doing this, I added some code to sort the branches for standard
layout repos with the Subvertpy backend (which gives us dirents in ctime order
instead of lexical order).

Testing done:
Connected the Apache ASF subversion repo (which has a bunch of projects in the
root) and loaded the new review request page. Saw that I got an alphabetized
list of projects, and could browse the commits on each of those projects as
expected. Verified with both PySVN and Subvertpy backends.

Reviewed at https://reviews.reviewboard.org/r/5486/