changelog: update read pending documentation

The pending index contains a full copy of the index + in-transaction data. We
replace "extend" with "overwrite" to make this clearer.

mercurial/changelog.py

@@ -269,7 +269,7 b' class changelog(revlog.revlog):'
         access its content.
 
         This function will read all the index data out of the pending file and
-        extend the main index."""
+        overwrite the main index."""
 
         if not self.opener.exists(file):
             return # no pending data for changelog