Merge pull request #5521 from sipsma/improve-contenthash-handle-change #5440

Job	Run time
build (windows/amd64)	1m 57s
build (freebsd/amd64)	2m 26s
test-freebsd-amd64	2m 41s
test-windows-amd64 (containerd, ./client)	3m 8s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 46s
test-windows-amd64 (containerd, ./worker/containerd)	2m 17s
test-windows-amd64 (containerd, ./solver)	2m 40s
test-windows-amd64 (containerd, ./frontend)	1m 56s
test-windows-amd64 (containerd, ./frontend/dockerfile)	33m 21s
test-windows-amd64 (./..., 1)	3m 34s
	56m 46s

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