Before executing svnserve, change the CWD to a readable directory

Summary: Fixes T10941. This avoids a confusing dead end when configuring Subversion hosting, where svnserve will fail to execute hooks if the CWD isn't readable by the vcs-user.

Test Plan:

• Updated and committed in a hosted SVN repository.
• Ran some git operations, too.
• @dpotter confirmed this locally in T10941.

Reviewers: chad

Reviewed By: chad

Subscribers: dpotter

Maniphest Tasks: T10941

Differential Revision: https://secure.phabricator.com/D15879