Add sfdupes implementation: scan, report, and trees subcommands

Makefile

@@ -0,0 +1,17 @@
+# sfdupes is pure Go; disable cgo so builds never touch the host C
+# toolchain.
+export CGO_ENABLED = 0
+
+.PHONY: all build check clean
+
+all: build
+
+build:
+	go build
+
+check: build
+	test -z "$$(gofmt -l .)"
+	go vet ./...
+
+clean:
+	rm -f sfdupes files.dat