[svn] Fix removing a binary file without svn:mime-type set

Summary:
If we're removing a binary file that didn't have svn:mime-type set properly,
we can't propset it (because the file doesn't exist locally). Instead, just
return a synthetic diff for the removed file.

Test Plan:
run arc diff in an svn working copy where I ran svn rm on a binary file that
doesn't have svn:mime-type set, and the diff correctly gets uploaded to
phabricator instead of erroring when trying to set properties.

Reviewers: epriestley, wez

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5655