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>

sha1_name.c

@@ -517,12 +517,21 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		}
 		if (read_ref_at(real_ref, at_time, nth, sha1, NULL,
 				&co_time, &co_tz, &co_cnt)) {
+			if (!len) {
+				if (!prefixcmp(real_ref, "refs/heads/")) {
+					str = real_ref + 11;
+					len = strlen(real_ref + 11);
+				} else {
+					/* detached HEAD */
+					str = "HEAD";
+					len = 4;
+				}
+			}
 			if (at_time)
 				warning("Log for '%.*s' only goes "
 					"back to %s.", len, str,
 					show_date(co_time, co_tz, DATE_RFC2822));
 			else {
-				free(real_ref);
 				die("Log for '%.*s' only has %d entries.",
 				    len, str, co_cnt);
 			}