Say which command shadows a key binding

* src/keymap.c (describe_vector): Say which command shadows this
binding.  (Bug#9293)
* test/src/keymap-tests.el
(help--describe-vector/bug-9293-one-shadowed-in-range): Adapt
test.
1 job for master in 10 minutes and 33 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #8931
test-all

00:10:33

 
Name Stage Failure
failed
test-all Test
  INFO     Processing OKURI-NASI entries...80% 
INFO Processing OKURI-NASI entries...90%
INFO Processing OKURI-NASI entries...100%
INFO Processing OKURI-NASI entries...done
make[4]: Leaving directory '/builds/emacs/emacs/leim'
make -C ../admin/grammars all EMACS="../../src/bootstrap-emacs"
make[4]: Entering directory '/builds/emacs/emacs/admin/grammars'
GEN ../../lisp/cedet/semantic/bovine/c-by.el
ERROR: Job failed: exit code 137