git: allow cloning commit shas not referenced by branch/tag #4644

Job	Run time
prepare	1s
test / prepare	2m 10s
test / run (containerd, ./frontend/dockerfile, integration)	5m 13s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 0s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 2s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	7m 25s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 4s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	11m 17s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 6s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	9m 0s
test / run (oci, ./frontend/dockerfile, integration)	5m 16s
test / run (oci, ./frontend/dockerfile, dockerfile)	11m 59s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 35s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 54s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	5m 7s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 57s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 13s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 7s
image (mainline)	5m 0s
image (labs)	5m 6s
release	0s
scout	0s
	1h 58m 32s

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