tools/git-utils.sh: add second "ref exists?" check
tools/git-utils.sh: add second "ref exists?" check

A while ago we moved from using `git rev-parse` to using `git fsck` to
check if we had a particular commit downloaded in order to avoid the
prolbme of having a false positive in the case where the commit object
had been downloaded but the file objects were missing (ie: interrupted
incomplete download).

Very old versions of git (such as that found in docker.io/node:14) have
a bug in `git fsck` where success can be returned even if the given
objects do not exist.  For that case, add a check using "git show",
which must also pass.

mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit