Hitting this issue myself now. I don’t quite understand git’s object system, but on both a failed build server I ssh’d into and a fresh clone my local machine I get:
$ ls .git/objects
info pack
So it seems no objects exist? Well, there are definitely objects, git rev-list --objects --all returns lots, but git rev-list --objects --all | grep $CIRCLE_SHA1 returns nothing. (And for the record $CIRCLE_SHA is the same hash value from the fatal: Could not parse object [...] error.)