checkout: tweak detached-orphan warning format

checkout: tweak detached-orphan warning format

When orphaning a commit on a detached HEAD, the warning
currently looks like:

  Warning: you are leaving 3 commits behind, not connected to
  any of your branches:

   - commit subject 1
   - commit subject 2
   - commit subject 3

  If you want to keep them by creating a new branch, this
  may be a good time to do so with:

     git branch new_branch_name 933a615ab0bc566dcfd8c01ec8af159f770d3fe5

Instead of using the "-" list, let's provide a more
traditional oneline format, with the abbreviated sha1 before
each subject. Users are accustomed to seeing commits in this
format, and having the sha1 of each commit can be useful if
you want to cherry-pick instead of creating a new branch.

The new format looks like:

  Warning: you are leaving 3 commits behind, not connected to
  any of your branches:

    933a615 commit subject 1
    824fcde commit subject 2
    fa49b1a commit subject 3

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