"rev-parse --verify --quiet $name" is meant to quietly exit with a
non-zero status when $name is not a valid object name, but still
gave error messages in some cases.

* da/rev-parse-verify-quiet:
  stash: prefer --quiet over shell redirection of the standard error stream
  refs: make rev-parse --quiet actually quiet
  t1503: use test_must_be_empty
  Documentation: a note about stdout for git rev-parse --verify --quiet