Improve readability

As suggested by @bboreham

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

model/labels/labels_stringlabels.go

@@ -364,8 +364,10 @@ func Compare(a, b Labels) int {
 	// Now we know that there is some difference before the end of a and b.
 	// Go back through the fields and find which field that difference is in.
 	firstCharDifferent, i := i, 0
-	for size, nextI := decodeSize(a.data, i); nextI+size <= firstCharDifferent; size, nextI = decodeSize(a.data, i) {
+	size, nextI := decodeSize(a.data, i)
+	for nextI+size <= firstCharDifferent {
 		i = nextI + size
+		size, nextI = decodeSize(a.data, i)
 	}
 	// Difference is inside this entry.
 	aStr, _ := decodeString(a.data, i)