store: introduce a main_file_path method for revlog

This help code that need to point revlog to an index file. This is put to use
in the upgrade code.

mercurial/store.py

@@ -524,6 +524,10 b' class RevlogStoreEntry(BaseStoreEntry):'
         self.target_id = target_id
         self.is_revlog_main = is_revlog_main
 
+    def main_file_path(self):
+        """unencoded path of the main revlog file"""
+        return self.unencoded_path
+
 
 @attr.s(slots=True)
 class StoreFile:

mercurial/upgrade_utils/engine.py

@@ -90,7 +90,7 b' def _copyrevlog(tr, destrepo, oldrl, ent'
         util.copyfile(olddata, newdata)
 
     if entry.revlog_type & store.FILEFLAGS_FILELOG:
-        unencodedname = entry.unencoded_path
+        unencodedname = entry.main_file_path()
         destrepo.svfs.fncache.add(unencodedname)
         if copydata:
             destrepo.svfs.fncache.add(unencodedname[:-2] + b'.d')
@@ -132,7 +132,7 b' def _perform_clone('
 ):
     """returns the new revlog object created"""
     newrl = None
-    revlog_path = entry.unencoded_path
+    revlog_path = entry.main_file_path()
     if matchrevlog(upgrade_op.revlogs_to_process, entry.revlog_type):
         ui.note(
             _(b'cloning %d revisions from %s\n')