R1:89f9f97159b7

Provide basic support for Subversion revprops

Summary:
Ref T11208. See that task for a more detailed description of revprops.

This allows revprop changes in a hosted Subversion repository if the repository has the "allow dangerous changes" flag set.

In the future, we could expand this into real Herald support, but the only use case we have for now is letting `svnsync` work.

Test Plan:
Edited revprops with `svn propset --revprop -r 2 propkey propvalue repositoryuri`:

- Tried before patch, got a…