Fix "checkout --track -b newbranch" on detached HEAD

The test to make sure that checkout fails when --track was asked for and
we cannot set up tracking information in t7201 was wrong, and it turns out
that the implementation for that feature itself was buggy.  This fixes it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -129,7 +129,9 @@ void create_branch(const char *head,
 			die("Cannot setup tracking information; starting point is not a branch.");
 		break;
 	case 1:
-		/* Unique completion -- good */
+		/* Unique completion -- good, only if it is a real ref */
+		if (track == BRANCH_TRACK_EXPLICIT && !strcmp(real_ref, "HEAD"))
+			die("Cannot setup tracking information; starting point is not a branch.");
 		break;
 	default:
 		die("Ambiguous object name: '%s'.", start_name);

t/t7201-co.sh

@@ -330,12 +330,14 @@ test_expect_success \
     test "$(git config branch.track2.merge)"
     git config branch.autosetupmerge false'
 
-test_expect_success \
-    'checkout w/--track from non-branch HEAD fails' '
-    git checkout -b delete-me master &&
-    rm .git/refs/heads/delete-me &&
-    test refs/heads/delete-me = "$(git symbolic-ref HEAD)" &&
-    test_must_fail git checkout --track -b track'
+test_expect_success 'checkout w/--track from non-branch HEAD fails' '
+    git checkout master^0 &&
+    test_must_fail git symbolic-ref HEAD &&
+    test_must_fail git checkout --track -b track &&
+    test_must_fail git rev-parse --verify track &&
+    test_must_fail git symbolic-ref HEAD &&
+    test "z$(git rev-parse master^0)" = "z$(git rev-parse HEAD)"
+'
 
 test_expect_success 'checkout an unmerged path should fail' '
 	rm -f .git/index &&