convert: use a collections.deque

This function was doing a list.pop(0) on a list whose length
was the number of revisions to convert. Popping an early element
from long lists is not an efficient operation.

collections.deque supports efficient inserts and pops at both
ends. So we switch to that data structure.

When converting the mozilla-unified repository, which has 445,748
revisions, this change makes the "sorting..." step of
`hg convert --sourcesort` significantly faster:

before: ~59.2s
after:   ~1.3s

Differential Revision: https://phab.mercurial-scm.org/D1934

hgext/convert/convcmd.py

@@ -6,6 +6,7 b''
 # GNU General Public License version 2 or any later version.
 from __future__ import absolute_import
 
+import collections
 import os
 import shlex
 import shutil
@@ -290,13 +291,13 b' class converter(object):'
             revisions without parents. 'parents' must be a mapping of revision
             identifier to its parents ones.
             """
-            visit = sorted(parents)
+            visit = collections.deque(sorted(parents))
             seen = set()
             children = {}
             roots = []
 
             while visit:
-                n = visit.pop(0)
+                n = visit.popleft()
                 if n in seen:
                     continue
                 seen.add(n)