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).

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.