davautocheck.sh: When running under something other than bash, teach the
prompt to exit as soon as something is available on stdin.  (Previously,
on FreeBSD, just answering "y\n" to the prompt would not be immediately
recognized.)

Thanks to gstein and peters for feedback.

* subversion/tests/cmdline/davautocheck.sh
  (query): Juggle a little to ensure that the terminal is in cbreak mode for
    the propmt.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507851 13f79535-47bb-0310-9956-ffa450edef68