Rename _must_refresh -> must_ensure

Remove the underscore since it's accessed in the manager class.

pythonx/UltiSnips/snippet/source/base.py

@@ -15,7 +15,7 @@ class SnippetSource:
     def __init__(self):
         self._snippets = defaultdict(SnippetDictionary)
         self._extends = defaultdict(set)
-        self._must_refresh = True
+        self.must_ensure = True
 
     def ensure(self, filetypes):
         """Ensures that snippets are loaded."""

pythonx/UltiSnips/snippet/source/file/base.py

@@ -27,11 +27,11 @@ def __init__(self):
         SnippetSource.__init__(self)
 
     def ensure(self, filetypes):
-        if self._must_refresh:
+        if self.must_ensure:
             for ft in self.get_deep_extends(filetypes):
                 if self._needs_update(ft):
                     self._load_snippets_for(ft)
-            self._must_refresh = False
+            self.must_ensure = False
 
     def refresh(self):
         self.__init__()

pythonx/UltiSnips/snippet_manager.py

@@ -982,7 +982,7 @@ def _check_filetype(self, ft):
         if ft not in self._filetypes:
             self._filetypes.append(ft)
             for _, source in self._snippet_sources:
-                source._must_refresh = True
+                source.must_ensure = True
 
 
 UltiSnips_Manager = SnippetManager(  # pylint:disable=invalid-name