git: allow scp-style urls without username #4794

Job	Run time
build (windows/amd64)	1m 50s
build (freebsd/amd64)	1m 33s
test-freebsd-amd64	20m 6s
test-windows-amd64 (containerd, ./client)	4m 21s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 3s
test-windows-amd64 (containerd, ./worker/containerd)	4m 12s
test-windows-amd64 (containerd, ./solver)	1m 31s
test-windows-amd64 (containerd, ./frontend)	3m 17s
test-windows-amd64 (containerd, ./frontend/dockerfile)	24m 23s
test-windows-amd64 (./..., 1)	11m 56s
	1h 16m 12s

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