Change _handle_empty_files() to call _run_svn() rather than execute.
Change _handle_empty_files() to call _run_svn() rather than execute.

In svn.py, _handle_empty_files() directly calls execute(diff_cmd,...) to invoke the 'svn diff' command.  However this does not operate as expected because the diff_cmd list does not contain 'svn' as the first entry.  This leads to an error message when running 'rbt diff':

$ rbt diff -d
>>> RBTools 0.7.1 alpha 0 (dev)
>>> Python 2.7.5 (default, Jun 17 2014, 18:11:42)
[GCC 4.8.2 20140120 (Red Hat 4.8.2-16)]
>>> Running on Linux-3.10.0-123.13.2.el7.x86_64-x86_64-with-centos-7.0.1406-Core
>>> Home = /home/gmyers
>>> Current directory = /home/gmyers/wc3
>>> Checking for a Subversion repository...
>>> Running: svn info --non-interactive
>>> Running: diff --version
>>> repository info: Path: file:///home/gmyers/svn_repo/myrepo, Base path: /,
Supports changesets: False
>>> Making HTTP GET request to http://localhost:8080/api/
>>> Cached response for HTTP GET http://localhost:8080/api/ expired and was
modified
>>> Running: svn info --non-interactive
>>> Running: diff --version
>>> repository info: Path: file:///home/gmyers/svn_repo/myrepo, Base path: /,
Supports changesets: False
>>> Running: svn status --ignore-externals
>>> Running: svn info .
>>> Running: svn diff --diff-cmd=diff --notice-ancestry -r BASE
>>> Running: svn info trunk/a.txt
>>> Running: svn info trunk
>>> Running: diff --diff-cmd=diff --notice-ancestry -r BASE --no-diff-deleted
>>> **Command exited with rc 2: ['diff', '--diff-cmd=diff', '--notice-ancestry',
'-r', 'BASE', '--no-diff-deleted']**
**diff: unrecognized option '--diff-cmd=diff'**
**diff: Try 'diff --help' for more information.**
<snip>

Fix is to switch from calling execute() to calling self._run_svn() following the recipe in a2890ae.  It appears that this single instance of execute() was overlooked between a2890ae and bfec7b3.  _run_svn() will prepend 'svn' to command line resulting in the intended 'svn diff' actually being executed.

Testing Done:
Executed 'rbt diff -d' after change and error message ralted to calling diff with an unknown option goes away.

Reviewed at https://reviews.reviewboard.org/r/6875/

datjwu/rbtools,reviewboard/rbtools,haosdent/rbtools,reviewboard/rbtools,datjwu/rbtools,davidt/rbtools,beol/rbtools,beol/rbtools,davidt/rbtools,haosdent/rbtools,reviewboard/rbtools,beol/rbtools,datjwu/rbtools,halvorlu/rbtools,halvorlu/rbtools,davidt/rbtools,halvorlu/rbtools,haosdent/rbtools