restore the prefix on output from npm version <inc>

PR-URL: #2718
Credit: @nlf
Close: #2718
Reviewed-by: @wraithgar

lib/version.js

@@ -37,15 +37,21 @@ const version = async args => {
     case 0:
       return list()
     case 1:
-      return output(await libversion(args[0], {
-        ...npm.flatOptions,
-        path: npm.prefix,
-      }))
+      return version_(args)
     default:
       throw usage
   }
 }
 
+const version_ = async (args) => {
+  const prefix = npm.flatOptions.tagVersionPrefix
+  const version = await libversion(args[0], {
+    ...npm.flatOptions,
+    path: npm.prefix,
+  })
+  return output(`${prefix}${version}`)
+}
+
 const list = async () => {
   const results = {}
   const { promisify } = require('util')

test/lib/version.js

@@ -6,6 +6,7 @@ let result = []
 const noop = () => null
 const npm = {
   flatOptions: {
+    tagVersionPrefix: 'v',
     json: false,
   },
   prefix: '',
@@ -144,17 +145,20 @@ t.test('with one arg', t => {
       t.deepEqual(
         opts,
         {
+          tagVersionPrefix: 'v',
           json: false,
           path: '',
         },
         'should forward expected options'
       )
-      t.end()
+      return '4.0.0'
     },
   })
 
   version(['major'], err => {
     if (err)
       throw err
+    t.same(result, ['v4.0.0'], 'outputs the new version prefixed by the tagVersionPrefix')
+    t.end()
   })
 })