Let get_sha1() parse the @{-N} syntax, with docs and tests.
Note that while @{-1}^2, @{-2}~5 and such are supported, @{-1}@{1} is
currently not allowed.
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
