fix(diff): set option where for pacote

pacote expects a **where** option that sets the cwd for all its
operations, ref: https://github.com/npm/pacote#options

This change properly sets that option in libnpmdiff options that will
then properly forward it to pacote, this is specially important for when
reading local file system specs.

lib/diff.js

@@ -42,7 +42,11 @@ const diff = async (args) => {
   const [a, b] = await retrieveSpecs(specs)
   npmlog.info('diff', { src: a, dst: b })
 
-  const res = await libdiff([a, b], { ...npm.flatOptions, diffFiles: args })
+  const res = await libdiff([a, b], {
+    ...npm.flatOptions,
+    diffFiles: args,
+    where: where(),
+  })
   return output(res)
 }
 

test/lib/diff.js

@@ -946,12 +946,13 @@ t.test('first arg is a valid semver range', t => {
 
 t.test('first arg is an unknown dependency name', t => {
   t.test('second arg is a qualified spec', t => {
-    t.plan(3)
+    t.plan(4)
 
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar@latest', 'should set expected first spec')
       t.equal(b, 'bar@2.0.0', 'should set expected second spec')
       t.match(opts, npm.flatOptions, 'should forward flat options')
+      t.match(opts, { where: '.' }, 'should forward pacote options')
     }
 
     npm.flatOptions.diff = ['bar', 'bar@2.0.0']