SVN buildSyntheticAdditionDiff: exit sooner if path is a directory
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

phacility/arcanist,fivestars/arcanist,Khan/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,aik099/arcanist,uber/arcanist,codelabs-ch/arcanist,uber/arcanist,r4nt/arcanist,uber/arcanist,wikimedia/phabricator-arcanist,codelabs-ch/arcanist,aik099/arcanist,uber/arcanist,chundongwang/arcanist,chundongwang/arcanist,phacility/arcanist,chundongwang/arcanist,aik099/arcanist,phacility/arcanist,wikimedia/phabricator-arcanist,r4nt/arcanist,fivestars/arcanist,Khan/arcanist,folsom-labs/arcanist,r4nt/arcanist,haskell-infra/arcanist,codelabs-ch/arcanist,phacility/arcanist,codelabs-ch/arcanist,folsom-labs/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,fivestars/arcanist,aik099/arcanist,Khan/arcanist,Khan/arcanist,codelabs-ch/arcanist,haskell-infra/arcanist,phacility/arcanist,folsom-labs/arcanist,wikimedia/phabricator-arcanist,chundongwang/arcanist,folsom-labs/arcanist,fivestars/arcanist