filectx: add a rawflags method

The new method returns the low-level revlog flag. We already have "rawdata"
so a "rawflags" makes sense.

Both "rawflags" and "rawdata" will be used in a later patch.

mercurial/context.py

@@ -1141,6 +1141,10 b' class filectx(basefilectx):'
     def rawdata(self):
         return self._filelog.revision(self._filenode, raw=True)
 
+    def rawflags(self):
+        """low-level revlog flags"""
+        return self._filelog.flags(self._filerev)
+
     def data(self):
         try:
             return self._filelog.read(self._filenode)