Revert "Cirrus-CI: Use the default Git history depth"

This reverts commit fcb4797c90f3f62a18b67542ae5b576a1271e6d4.
With CIRRUS_CLONE_DEPTH unspecified (using the default of 50) the
time to clone the repo increased from a little under 2 minutes to over
6 minutes, possibly due to additional processing required by GitHub.

We will want to watch out for occasional `Failed to force reset to
SHA: object not found!` errors.