Use a different legacy id to assume a non existent change

So this test assumes change 12345 to be an invalid change (it's
a pretty huge number after all ;) but today is the day where this change
came into existence.

For now increase the number a bit. Increase
_MAXIMUM_GERRIT_NUMBER_LENGTH by one so we can have up to 1M changes,
giving us a little more headroom.

Change-Id: I1124442cf7fa74a470bb3cce8b185cdd651de5de
Reviewed-on: https://chromium-review.googlesource.com/286548
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Dave Borowitz <dborowitz@google.com>
Tested-by: Dave Borowitz <dborowitz@google.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
3 files changed