censor: use a reasonable amount of memory

Before this change, trying to censor some random revision uses an ever
increasing amount of memory (I stopped at 20GB, but it was by no means
finished), presumably because these contexts have a lot of
information that is kept alive.
After this change, the memory usage plateaus quickly.

Differential Revision: https://phab.mercurial-scm.org/D4582

hgext/censor.py

@@ -83,8 +83,11 b" def _docensor(ui, repo, path, rev='', to"
         raise error.Abort(_('file does not exist at revision %s') % rev)
 
     fnode = fctx.filenode()
-    headctxs = [repo[c] for c in repo.heads()]
-    heads = [c for c in headctxs if path in c and c.filenode(path) == fnode]
+    heads = []
+    for headnode in repo.heads():
+        c = repo[headnode]
+        if path in c and c.filenode(path) == fnode:
+            heads.append(c)
     if heads:
         headlist = ', '.join([short(c.node()) for c in heads])
         raise error.Abort(_('cannot censor file in heads (%s)') % headlist,