for-each-ref: handle multiline subjects like --pretty

for-each-ref: handle multiline subjects like --pretty

Generally the format of a git tag or commit message is:

  subject

  body body body
  body body body

However, we occasionally see multiline subjects like:

  subject
  with multiple
  lines

  body body body
  body body body

The rest of git treats these multiline subjects as something
to be concatenated and shown as a single line (e.g., "git
log --pretty=format:%s" will do so since f53bd74). For
consistency, for-each-ref should do the same with its
"%(subject)".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>