dagop: copy basefilectx.ancestors() to free function

The primary goal of this series is to make follow() support multiple start
revisions.

dagop.filectxancestors() will be extended to take multiple filectxs.
basefilectx.ancestors() is not forwarded to this function because doing that
would resurrect the performance issue fixed by 24b57c3899f8.

mercurial/dagop.py

@@ -75,6 +75,23 b' def _walkrevtree(pfunc, revs, startdepth'
                 if prev != node.nullrev:
                     heapq.heappush(pendingheap, (heapsign * prev, pdepth))
 
+def filectxancestors(fctx, followfirst=False):
+    """Like filectx.ancestors()"""
+    visit = {}
+    c = fctx
+    if followfirst:
+        cut = 1
+    else:
+        cut = None
+
+    while True:
+        for parent in c.parents()[:cut]:
+            visit[(parent.linkrev(), parent.filenode())] = parent
+        if not visit:
+            break
+        c = visit.pop(max(visit))
+        yield c
+
 def _genrevancestors(repo, revs, followfirst, startdepth, stopdepth, cutfunc):
     if followfirst:
         cut = 1

mercurial/revset.py

@@ -930,7 +930,8 b' def _follow(repo, subset, x, name, follo'
         s = set()
         for fname in files:
             fctx = c[fname]
-            s = s.union(set(c.rev() for c in fctx.ancestors(followfirst)))
+            a = dagop.filectxancestors(fctx, followfirst)
+            s = s.union(set(c.rev() for c in a))
             # include the revision responsible for the most recent version
             s.add(fctx.introrev())
     else: