R1:b2e89a9e4876

Fix several error handling issues with Subversion commits in Diffusion

Summary:
Ref T9513. I checked this briefly but didn't do a very thorough job of it.

- Don't try to query merges for Subversion, since it doesn't support them.
- Fix up "existsquery" to work properly (and efficiently) for both hosted and imported repositories.
- Fix up "parentsquery" to have similar behavior on invalid commits to other VCSes (throw an exception).

Test Plan:
- No more merges warning on SVN.
- Hosted SVN gets…