rev-parse: tests git rev-parse --verify master@{n}, for various n

rev-parse: tests git rev-parse --verify master@{n}, for various n

This commit introduces tests that verify that rev-parse
parses master@{n} correctly for various values of n less
than, equal to and greater than the number of revisions
in the reference log.

In particular, these tests check that rev-parse exits with a
non-zero status code and prints a message of the
following form to stderr.

    fatal: Log for [^ ]* only has [0-9][0-9]* entries.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>