rename: add hint about --at-rev if source file doesn't exist

It's quite common that users want to record copy (rename) information
after committing the working copy changes (i.e. an added and a deleted
file). When they try `hg mv [--after] <src> <dst>`, that just fails
because the source file doesn't exist. It seems helpful if we can
point them to `--at-rev=.` in this case.

Differential Revision: https://phab.mercurial-scm.org/D10697

mercurial/cmdutil.py

@@ -1851,7 +1851,10 b' def copy(ui, repo, pats, opts, rename=Fa'
             continue
         copylist.append((tfn(pat, dest, srcs), srcs))
     if not copylist:
-        raise error.InputError(_(b'no files to copy'))
+        hint = None
+        if rename:
+            hint = _(b'maybe you meant to use --after --at-rev=.')
+        raise error.InputError(_(b'no files to copy'), hint=hint)
 
     errors = 0
     for targetpath, srcs in copylist:

tests/test-copy.t

@@ -115,6 +115,7 b' should fail - foo is not managed'
   $ hg mv foo bar
   foo: not copying - file is not managed
   abort: no files to copy
+  (maybe you meant to use --after --at-rev=.)
   [10]
   $ hg st -A
   ? foo
@@ -124,14 +125,17 b' respects ui.relative-paths'
   $ hg mv ../foo ../bar
   ../foo: not copying - file is not managed
   abort: no files to copy
+  (maybe you meant to use --after --at-rev=.)
   [10]
   $ hg mv ../foo ../bar --config ui.relative-paths=yes
   ../foo: not copying - file is not managed
   abort: no files to copy
+  (maybe you meant to use --after --at-rev=.)
   [10]
   $ hg mv ../foo ../bar --config ui.relative-paths=no
   foo: not copying - file is not managed
   abort: no files to copy
+  (maybe you meant to use --after --at-rev=.)
   [10]
   $ cd ..
   $ rmdir dir

tests/test-remotefilelog-prefetch.t

@@ -237,6 +237,7 b''
   $ hg mv z2 z3
   z2: not copying - file is not managed
   abort: no files to copy
+  (maybe you meant to use --after --at-rev=.)
   [10]
   $ find $CACHEDIR -type f | sort
 .. The following output line about files fetches is globed because it is