Fix AttributeError thrown when no extensions were detected (Fixes #91).

The bug was introduced with the commit ecc67a3 and was caused by the
initialization of an empty Changes object.

This fix makes sure that the __iadd__() method properly handles empty
change sets and returns a satisfactory object back when completing
execution.

gitinspector/changes.py

@@ -239,6 +239,8 @@ class Changes(object):
 
 			for commit in other.commits:
 				bisect.insort(self.commits, commit)
+			if not self.commits and not other.commits:
+				self.commits = []
 
 			return self
 		except AttributeError: