t1100-*.sh: Fix an intermittent test failure

t1100-*.sh: Fix an intermittent test failure

In particular, the final test ('flags and then non flags') fails
intermittently, depending on how much time elapsed between the
invocations of "git commit-tree" when creating the commits which
later have their commit id's compared. For example, if the commits
for childid-3 and childid-4 are created 1 or more seconds apart,
then the commits, which would otherwise be identical, will have
different commit id's.

In order to make the test reproducible, we remove the variability
by setting the author and committer times to a well defined state.
We accomplish this with a single call to 'test_tick' at the start
of the test.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>