copy: give better error message when no source paths found with --at-rev

The new error message matches what we show when marking copies in the
working copy.

Differential Revision: https://phab.mercurial-scm.org/D8496

mercurial/cmdutil.py

@@ -1552,6 +1552,8 b' def copy(ui, repo, pats, opts, rename=Fa'
             for abs, rel, exact in srcs:
                 copylist.append(abs)
 
+        if not copylist:
+            raise error.Abort(_(b'no files to copy'))
         # TODO: Add support for `hg cp --at-rev . foo bar dir` and
         # `hg cp --at-rev . dir1 dir2`, preferably unifying the code with the
         # existing functions below.

tests/test-rename-rev.t

@@ -25,10 +25,9 b' Test single file'
   abort: --at-rev requires --after
   [255]
 # Errors out with non-existent source
-BROKEN: this should have a better error message
   $ hg cp -A --at-rev . d1/non-existent d1/d
   d1/non-existent: no such file in rev 55d1fd85ef0a
-  abort: --at-rev requires a single source
+  abort: no files to copy
   [255]
 # Errors out with non-existent destination
   $ hg cp -A --at-rev . d1/b d1/non-existent