t1200: fix a timing dependent error

t1200: fix a timing dependent error

The fourth test of show-branch in t1200 test was failing but only
sometimes. It only failed when two commits created in an earlier
test had different timestamps. When they were created within the
same second, the actual output matched the expected output.

Fix this by using test_tick to force reliable timestamps and update
the expected output so it does not to depend on the commits made in
the same sacond.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>