tests: at-combinations: @{N} versus HEAD@{N}

All the tests so far check that @{N} is the same as HEAD@{N} (for
positive N). However, this is not always the case; write a couple of
tests for this.

[fc: simplify some wording]

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1508-at-combinations.sh

@@ -61,4 +61,17 @@ nonsense "@{1}@{u}"
 nonsense "HEAD@{-1}"
 nonsense "@{-1}@{-1}"
 
+# @{N} versus HEAD@{N}
+
+check "HEAD@{3}" commit old-two
+nonsense "@{3}"
+
+test_expect_success 'switch to old-branch' '
+	git checkout old-branch
+'
+
+check HEAD ref refs/heads/old-branch
+check "HEAD@{1}" commit new-two
+check "@{1}" commit old-one
+
 test_done