Make PrePushHook properly terminate ref lines

Make PrePushHook properly terminate ref lines

All of the input lines passed to pre-push hook scripts must be properly
terminated by '\n', so that normal shell scripts like the git-supplied
pre-push.sample work properly, even when pushing just a single branch.

With the old code, hook scripts that use the following pattern didn't
process the last line, because 'read' has a non-zero exit status when
EOF is encountered:
  while read local_ref local_sha remote_ref remote_sha; do ... done

Change-Id: Id899662ed3fedef6c314fc4b2ddf91a6dcb98cbb
Signed-off-by: Markus Keller <markus.kell.r@gmail.com>