store: only access is_volatile information through the file object

This make sure other code only access this information through the proper API,
and it prepare for store entries to be able to agregate multiple files.

mercurial/store.py

@@ -460,7 +460,7 b' class BaseStoreEntry:'
     This is returned by `store.walk` and represent some data in the store."""
 
     unencoded_path = attr.ib()
-    is_volatile = attr.ib(default=False)
+    _is_volatile = attr.ib(default=False)
     _file_size = attr.ib(default=None)
 
     def __init__(
@@ -470,7 +470,7 b' class BaseStoreEntry:'
         file_size=None,
     ):
         self.unencoded_path = unencoded_path
-        self.is_volatile = is_volatile
+        self._is_volatile = is_volatile
         self._file_size = file_size
 
     def files(self):
@@ -478,7 +478,7 b' class BaseStoreEntry:'
             StoreFile(
                 unencoded_path=self.unencoded_path,
                 file_size=self._file_size,
-                is_volatile=self.is_volatile,
+                is_volatile=self._is_volatile,
             )
         ]