Auto merge of #177 - Manishearth:try-fix, r=jdm

Use None, not False, for try_choose default value

False gets stored as 0

fixes #176

I also ran `update table pull set try_choose=null where try_choose=0;`
on the database to clean up things affected by this.

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Author: bors-servo

homu/action.py

@@ -204,7 +204,7 @@ def review_approved(state, realtime, approver, username,
     if sha_cmp(sha, state.head_sha):
         state.approved_by = approver
         state.try_ = False
-        state.try_choose = False
+        state.try_choose = None
         state.set_status('')
 
         state.save()