dagop: raise ProgrammingError if stopdepth < 0

revset.py should never send such a value.

mercurial/dagop.py

@@ -32,8 +32,10 b' def _genrevancestors(repo, revs, followf'
         startdepth = 0
     if stopdepth is None:
         stopdepth = _maxlogdepth
-    if stopdepth <= 0:
+    if stopdepth == 0:
         return
+    if stopdepth < 0:
+        raise error.ProgrammingError('negative stopdepth')
 
     cl = repo.changelog