Repository: make use of peel for diff()

Instead of trying to reimplement parts of it, make use of Object.peel()
to get to a Tree, with short-circuit returns for None and Blob which can
be handled differently.

pygit2/repository.py

@@ -375,20 +375,20 @@ def diff(self, a=None, b=None, cached=False, flags=GIT_DIFF_NORMAL,
         API (Tree.diff_to_tree()) directly.
         """
 
-        def treeish_to_tree(obj):
-            try:
+        def tree_or_blob(obj):
+            if obj is None:
+                return obj
+
+            if is_string(obj):
                 obj = self.revparse_single(obj)
-            except:
-                pass
 
-            if isinstance(obj, Commit):
-                return obj.tree
-            elif isinstance(obj, Reference):
-                oid = obj.resolve().target
-                return self[oid]
+            if isinstance(obj, Blob):
+                return obj
+
+            return obj.peel(Tree)
 
-        a = treeish_to_tree(a) or a
-        b = treeish_to_tree(b) or b
+        a = tree_or_blob(a)
+        b = tree_or_blob(b)
 
         opt_keys = ['flags', 'context_lines', 'interhunk_lines']
         opt_values = [flags, context_lines, interhunk_lines]