SVN buildSyntheticAdditionDiff: exit sooner if path is a directory

Summary:
When doing svn copy, or svn mv, a SynthenticAdditionDiff is generated.

If the path is a directory, an error will occur when checking the
mime-type of the directory. Immediately after the properties check,
the function returns null if the path is a directory. Move this
check to before the properties check to avoid exiting with an error.

Command failed with error #1!
COMMAND
svn propget 'svn:mime-type' '/home/trasz/svn/ports/cad/py-pycam'@

STDOUT
(empty)

STDERR
svn: warning: W200017: Property 'svn:mime-type' not found on '/home/trasz/svn/ports/cad/py-pycam@'
svn: E200000: A problem occurred; see other errors for details

(Run with `--trace` for a full exception trace.)

Test Plan: Created differentials of changes with svn copy and svn mv

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Tags: #subversion

Differential Revision: https://secure.phabricator.com/D15985