Introduce new pretty formats %g[sdD] for reflog information

Introduce new pretty formats %g[sdD] for reflog information

Add three new --pretty=format escapes:

  %gD  long  reflog descriptor (e.g. refs/stash@{0})
  %gd  short reflog descriptor (e.g. stash@{0})
  %gs  reflog message

This is achieved by passing down the reflog info, if any, inside the
pretty_print_context struct.

We use the newly refactored get_reflog_selector(), and give it some
extra functionality to extract a shortened ref.  The shortening is
cached inside the commit_reflogs struct; the only allocation of it
happens in read_complete_reflog(), where it is initialised to 0.  Also
add another helper get_reflog_message() for the message extraction.

Note that the --format="%h %gD: %gs" tests may not work in real
repositories, as the --pretty formatter doesn't know to leave away the
": " on the last commit in an incomplete (because git-gc removed the
old part) reflog.  This equivalence is nevertheless the main goal of
this patch.

Thanks to Jeff King for reviews, the %gd testcase and documentation.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>