Clean git repo before checkout

Sometimes git repo become unusable - it contains a lot of
untracked files. Due to this the following checkout fails
and none of commits processed. The solution is to reset the
state and clean all untracked files.

Change-Id: Id230147ac3bfe739ad8be45a7f87da59807fcea4

stackalytics/processor/vcs.py

@@ -95,6 +95,8 @@ class Git(Vcs):
 
     def _checkout(self, branch):
         try:
+            sh.git('clean', '-d', '--force')
+            sh.git('reset', '--hard')
             sh.git('checkout', 'origin/' + branch)
             return True
         except sh.ErrorReturnCode as e: