fix: relative() helper with --link-prefix

The previous implementation did not work when the link prefix
went back deeper than the current folder allows
e.g. ../../.. when running from /app

Which is expected to work regardless because the link prefix
is assumed to exist relative to the final deployment

Fixes #351

src/website/theme-base/helpers/relative.js

@@ -6,7 +6,8 @@ module.exports = (target, options) => {
     return target
   }
   const albumPath = options.data.root.album.path
-  const relative = path.relative(path.dirname(albumPath), target)
+  const backToGalleryRoot = path.relative(path.dirname(albumPath), '.')
+  const relative = path.join(backToGalleryRoot, target)
   const url = relative.replace(/\\/g, '/')
   // Escape single/double quotes
   return url.replace(/'/g, '%27').replace(/"/g, '%22')

test/themes/helpers/relative.spec.js

@@ -48,6 +48,26 @@ describe('Handlebars helpers: relative', () => {
       should(res).eql('<img src="../media/thumbs/img.jpg" />')
     })
 
+    it('can use a relative link from the root album', () => {
+      const template = handlebars.compile('<link rel="stylesheet" href="{{relative \'../../photos/img.jpg\'}}" />')
+      const res = template({
+        album: {
+          path: 'index.html'
+        }
+      })
+      should(res).eql('<link rel="stylesheet" href="../../photos/img.jpg" />')
+    })
+
+    it('can use relative link from a nested album', () => {
+      const template = handlebars.compile('<link rel="stylesheet" href="{{relative \'../../photos/img.jpg\'}}" />')
+      const res = template({
+        album: {
+          path: 'albums/holidays.html'
+        }
+      })
+      should(res).eql('<link rel="stylesheet" href="../../../photos/img.jpg" />')
+    })
+
     it('does not do anything if the path is an absolute URL', () => {
       // This can happen when using --link-prefix
       const url = 'http://example.com/photo.jpg'