extdata: just use iterator to read lines one by one

mercurial/scmutil.py

@@ -1052,7 +1052,7 b' def extdatasource(repo, source):'
         src = url.open(repo.ui, spec)
 
     try:
-        for l in src.readlines():
+        for l in src:
             if " " in l:
                 k, v = l.strip().split(" ", 1)
             else: