sha1_name: fix error message for @{u}

Currently, when no (valid) upstream is configured for a branch, you 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.

To justify that this change is safe, consider that all callers of
interpret_branch_name() have to fall in two categories:

1. Direct end-user facing applications like [rev-parse, show] calling in
   with end-user data (in which case the data can contain "@{u}").
   Failing immediately is the right thing to do: the only difference is
   that the die() happens in interpret_branch_name() instead of
   die_verify_filename(), and this is desirable.

2. Callers calling in with programmatic data, and expecting the function
   to return and not die().  In this case, why would anyone ever
   construct a string containing "@{u}" programmatically in the first
   place?  A grep reveals that no part of the code hard-codes either
   "@{u}" or "@{upstream}".  So, these callers will never hit the
   codepath touched by the patch.

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
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 ugly, inconsistent, and failing to 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.

Simple, consistent, and informative; suitable for output even from
plumbing commands like rev-parse.

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