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

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

  $ 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.
  ...

The empty string '' looks ugly and inconsistent with the output of
<branch>@{<N>}.  Replace it with the string 'current branch'.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>