copies: fix crash on in changeset-centric tracing from commit to itself

When we trace copies from a changeset to itself, the "work" queue ends
up empty and we hit the "assert False" after it.

It was only the last of the three added tests that failed before this
patch. That is because the other two cases have fast paths, so
_committedforwardcopies() is never reached.

Differential Revision: https://phab.mercurial-scm.org/D6675

mercurial/copies.py

@@ -246,7 +246,7 b' def _committedforwardcopies(a, b, base, '
     return cm
 
 def _changesetforwardcopies(a, b, match):
-    if a.rev() == node.nullrev:
+    if a.rev() in (node.nullrev, b.rev()):
         return {}
 
     repo = a.repo()

tests/test-copies.t

@@ -58,6 +58,17 b' Test filtering copies by path. We do fil'
   x -> y
   $ hg debugpathcopies 1 0 y
 
+Copies not including commit changes
+  $ newrepo
+  $ echo x > x
+  $ hg ci -Aqm 'add x'
+  $ hg mv x y
+  $ hg debugpathcopies . .
+  $ hg debugpathcopies . 'wdir()'
+  x -> y
+  $ hg debugpathcopies 'wdir()' .
+  y -> x
+
 Copy a file onto another file
   $ newrepo
   $ echo x > x