Sourcetree closes pageant on exit even if it hadn't been started by Sourcetree.

Issue reproduction:

1. Make sure that Options->General->'Automatically start SSH agent when Sourcetree opens' option is disabled and close Sourcetree.
2. Start external (=not provided with Sourcetree) pageant instance.
3. Start Sourcetree.
4. Close Sourcetree

Result:

Pageant is closed.

Expected behaviour:

Sourcetree should not stop pageant if it hadn't been started by Sourcetree. Pageant can be used by other applications.