fix: faster diff (#539)

Author: H4ad

functions/diff.js

@@ -2,11 +2,11 @@ const parse = require('./parse')
 const eq = require('./eq')
 
 const diff = (version1, version2) => {
-  if (eq(version1, version2)) {
+  const v1 = parse(version1)
+  const v2 = parse(version2)
+  if (eq(v1, v2)) {
     return null
   } else {
-    const v1 = parse(version1)
-    const v2 = parse(version2)
     const hasPre = v1.prerelease.length || v2.prerelease.length
     const prefix = hasPre ? 'pre' : ''
     const defaultResult = hasPre ? 'prerelease' : ''