sha1_name: fix error message for @{<N>}, @{<date>}

sha1_name: fix error message for @{<N>}, @{<date>}

Currently, when we try to resolve @{<N>} or @{<date>} when the reflog
doesn't go back far enough, we get errors like:

  # on branch master
  $ git show @{10000}
  fatal: Log for '' only has 7 entries.

  $ git show @{10000.days.ago}
  warning: Log for '' only goes back to Tue, 21 May 2013 14:14:45 +0530.
  ...

  # detached HEAD case
  $ git show @{10000}
  fatal: Log for '' only has 2005 entries.

  $ git show master@{10000}
  fatal: Log for 'master' only has 7 entries.

The empty string '' is confusing and does not convey information
about whose logs we are inspecting.  Change this so that we get:

  # on branch master
  $ git show @{10000}
  fatal: Log for 'master' only has 7 entries.

  $ git show @{10000.days.ago}
  warning: Log for 'master' only goes back to Tue, 21 May 2013 14:14:45 +0530.
  ...

  # detached HEAD case
  $ git show @{10000}
  fatal: Log for 'HEAD' only has 2005 entries.

  $ git show master@{10000}
  fatal: Log for 'master' only has 7 entries.

Also one of the message strings given to die() now points into
real_ref that was not used in that fashion, so stop freeing the
underlying storage for it.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Bug-spotted-and-fixed-by: Thomas Rast
Signed-off-by: Junio C Hamano <gitster@pobox.com>