revlog: use `_format_flags` to access flags instead of `header`

It seems better to reuse the variable we carefully extracted

This also open the way to more flexible way to retrieve these flags.

Differential Revision: https://phab.mercurial-scm.org/D10598

mercurial/revlog.py

@@ -510,8 +510,8 b' class revlog(object):'
                     % (flags >> 16, fmt, self.display_id)
                 )
 
-            self._inline = header & FLAG_INLINE_DATA
-            self._generaldelta = header & FLAG_GENERALDELTA
+            self._inline = self._format_flags & FLAG_INLINE_DATA
+            self._generaldelta = self._format_flags & FLAG_GENERALDELTA
 
         elif fmt == REVLOGV2:
             if flags & ~REVLOGV2_FLAGS: