Parse unusual Subversion protocol frames which contain extra whitespace

Summary:
Fixes T13140. See PHI660.

Recent versions of Subversion can send a (get-file true false false ) protocol frame with extra space between "false" and "false". This is allowed by the protocol spec, but never normally happens, and we do not parse it correctly.

Instead, parse it correctly.

Test Plan:

• Added unit tests.
• Ran svn proplist svn+ssh://.../diffusion/X/file.c under SVN 1.10 before and after the change.
  • Before: indefinite hang.
  • After: completed in finite time.

Reviewers: amckinley, asherkin

Reviewed By: amckinley, asherkin

Maniphest Tasks: T13140

Differential Revision: https://secure.phabricator.com/D19451