Cirrus-CI: Use the default Git history depth

Which is 50. I saw a few errors like
Failed to force reset to SHA: object not found! which seems is
happening because the SHA is not available because there were two
commits pushed almost simultaneously and the second from the top fails
with this error because the SHA is not in the history.

Pull Request: https://github.com/freebsd/freebsd-src/pull/454

Comments on re-commit from emaste:

Originally committed as fcb4797c90f3 and reverted in 80a840b8ba03 due
to the clone operation taking significantly longer. However, I have
seen many failures due to the "object not found" issue recently.
7 of 37 recent runs failed because of this, and intermittent failures
like this makes CI much less useful.

Prefer longer-running runs to intermittent failures.