t4200: make "rerere gc" test more robust

t4200: make "rerere gc" test more robust

The test blindly trusted that there may be _some_ entries left in
the rerere database, and used them by updating their timestamps to
see if the gc threshold variables are honoured correctly.  This
won't work if there is no entry in the database when the test
begins.

Instead, clear the rerere database, and populate it with a few known
entries (which are bogus, but for the purpose of testing "garbage
collection", it does not matter---we want to make sure we collect
old cruft, even if the files are corrupt rerere database entries),
and use them for the expiry test.

Signed-off-by: Junio C Hamano <gitster@pobox.com>