sha1_name: fix error message for @{u}

Currently, when no (valid) upstream is configured for a branch, we get
an error like:

  $ git show @{u}
  error: No upstream configured for branch 'upstream-error'
  error: No upstream configured for branch 'upstream-error'
  fatal: ambiguous argument '@{u}': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

The "error: " line actually appears twice, and the rest of the error
message is useless.  In sha1_name.c:interpret_branch_name(), there is
really no point in processing further if @{u} couldn't be resolved, and
we might as well die() instead of returning an error().  After making
this change, you get:

  $ git show @{u}
  fatal: No upstream configured for branch 'upstream-error'

Also tweak a few tests in t1507 to expect this output.

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