copy: to find copy source, walk parent of revision we're marking copies in

As shown in the previous patch, `hg cp --after --at-rev . src dst`
fails if `src` is not in `.`. It seems obvious that you should always
walk the *parent* of the revision you're marking copies in, but that's
not how it was done for the working copy, and I didn't think to change
it when marking copies in a non-working-copy commit.

This patch fixes that by walking the parent commit instead, but only
if we're marking copies for a non-working-copy commit. We need to
leave the working-copy code unchanged because it depends on the weird
behavior of `workingctx.walk()`. With these changes, there's very
little overlap between the working-copy version and the
non-working-copy version of `walkpats()`, but I've refrained from
cleaning that up on the stable branch.

Differential Revision: https://phab.mercurial-scm.org/D8494

mercurial/cmdutil.py

@@ -1496,8 +1496,11 b' def copy(ui, repo, pats, opts, rename=Fa'
 
     def walkpat(pat):
         srcs = []
-        m = scmutil.match(ctx, [pat], opts, globbed=True)
-        for abs in ctx.walk(m):
+        # TODO: Inline and simplify the non-working-copy version of this code
+        # since it shares very little with the working-copy version of it.
+        ctx_to_walk = ctx if ctx.rev() is None else pctx
+        m = scmutil.match(ctx_to_walk, [pat], opts, globbed=True)
+        for abs in ctx_to_walk.walk(m):
             rel = uipathfn(abs)
             exact = m.exact(abs)
             if abs not in ctx:

tests/test-rename-rev.t

@@ -47,13 +47,11 b' Test moved file (not copied)'
   $ hg add d1/d
   $ hg ci -m 'move d1/b to d1/d'
   created new head
-BROKEN: this should work
   $ hg cp -A --at-rev . d1/b d1/d
-  d1/b: no such file in rev 519850c3ea27
-  abort: --at-rev requires a single source
-  [255]
+  saved backup bundle to $TESTTMP/.hg/strip-backup/519850c3ea27-153c8fbb-copy.hg
   $ hg st -C --change .
   A d1/d
+    d1/b
   R d1/b
 
 Test using directory as destination