Ignore _profile_base_location_ when computing signatures

We want two identical sections at different URL to be able to produce
same signature. This is feature is useful for slapos.recipe.cmmi's
shared parts.

Rebase instructions:
- squash with "apply "Support ${:_profile_base_location_}."

src/zc/buildout/buildout.py

@@ -1316,9 +1316,16 @@ class Options(DictMixin):
             self.buildout._parts.append(name)
 
         m = md5()
+        # _profile_base_location_ is ignored in signatures, so that two sections
+        # at different URLs can have same signature
+        _profile_base_location_ = self.get('_profile_base_location_')
         # access values through .get() instead of .items() to detect unused keys
         for key in sorted(self.keys()):
+            if key == '_profile_base_location_':
+                continue
             value = self._data.get(key, self._cooked.get(key, self._raw.get(key)))
+            if _profile_base_location_:
+                value = value.replace(_profile_base_location_, '${:_profile_base_location_}')
             m.update(('%r\0%r\0' % (key, value)).encode())
         self.items_signature = '%s:%s' % (name, m.hexdigest())