Fix version comparison in upgrade

cmd/syncthing/upgrade_test.go

@@ -0,0 +1,27 @@
+package main
+
+import "testing"
+
+var testcases = []struct {
+	a, b string
+	r    int
+}{
+	{"0.1.2", "0.1.2", 0},
+	{"0.1.3", "0.1.2", 1},
+	{"0.1.1", "0.1.2", -1},
+	{"0.3.0", "0.1.2", 1},
+	{"0.0.9", "0.1.2", -1},
+	{"1.1.2", "0.1.2", 1},
+	{"0.1.2", "1.1.2", -1},
+	{"0.1.10", "0.1.9", 1},
+	{"0.10.0", "0.2.0", 1},
+	{"30.10.0", "4.9.0", 1},
+}
+
+func TestCompareVersions(t *testing.T) {
+	for _, tc := range testcases {
+		if r := compareVersions(tc.a, tc.b); r != tc.r {
+			t.Errorf("compareVersions(%q, %q): %d != %d", tc.a, tc.b, r, tc.r)
+		}
+	}
+}