Fix parsing of @{-1}@{1}

To do that, Git no longer looks forward for the '@{' corresponding to the
closing '}' but backward, and dwim_ref() as well as dwim_log() learnt
about the @{-<N>} notation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1505-rev-parse-last.sh

@@ -54,7 +54,7 @@ test_expect_success '@{-1}^2 works' '
 	test_rev_equivalent side^2 @{-1}^2
 '
 
-test_expect_failure '@{-1}@{1} works' '
+test_expect_success '@{-1}@{1} works' '
 	test_rev_equivalent side@{1} @{-1}@{1}
 '