Make relative_to_absolute more configurable

Changelog: changed

src/wobsite.inko

@@ -371,7 +371,7 @@ class UpdateAssetLinks {
 
   fn hashed_url(url: String) -> String {
     let key = if url.starts_with?('..') {
-      relative_to_absolute(@url, url)
+      relative_to_absolute(@url, url, as_file: true)
     } else {
       url
     }

src/wobsite/url.inko

@@ -79,14 +79,22 @@ fn pub file_url(directory: ref Path, path: ref Path) -> String {
   }
 }
 
-fn pub relative_to_absolute(current: String, url: String) -> String {
+# Converts a relative URL to an absolute URL, based on the current/source URL of
+# the document that links to the relative URL.
+#
+# The `current` argument specifies the URL of the document we are linking
+# _from_, while `url` specifies the relative URL linked _to_.
+#
+# If `as_file` is set to `true`, the last component of the target URL is treated
+# as if it were a file instead of a directory. This affects how many times we
+# have to move "up" in the returned relative URL.
+fn pub relative_to_absolute(
+  current: String,
+  url: String,
+  as_file: Bool,
+) -> String {
   let steps = url.split('/').take_while fn (v) { v == '..' }.count
-
-  # We add 1 here such that if the current URL is /foo/bar and we link to
-  # foo.css, the returned URL is /foo/foo.css, not /foo/bar/foo.css. In other
-  # words, the final component is treated as if it were a file instead of a
-  # directory.
-  let mut pending = steps + 1
+  let mut pending = steps + as_file.to_int
   let chunks = current.split('/').to_array
 
   while pending > 0 {

test/wobsite/test_url.inko

@@ -35,16 +35,29 @@ fn pub tests(t: mut Tests) {
   }
 
   t.test('url.relative_to_absolute') fn (t) {
-    t.equal(url.relative_to_absolute('/', '../style.css'), '/style.css')
-    t.equal(url.relative_to_absolute('/foo/', '../style.css'), '/style.css')
-    t.equal(url.relative_to_absolute('/foo/', 'style.css'), '/style.css')
     t.equal(
-      url.relative_to_absolute('/foo/bar', '../style.css'),
+      url.relative_to_absolute('/', '../style.css', as_file: true),
       '/style.css'
     )
     t.equal(
-      url.relative_to_absolute('/foo/bar/baz', '../style.css'),
+      url.relative_to_absolute('/foo/', '../style.css', as_file: true),
+      '/style.css'
+    )
+    t.equal(
+      url.relative_to_absolute('/foo/', 'style.css', as_file: true),
+      '/style.css'
+    )
+    t.equal(
+      url.relative_to_absolute('/foo/bar', '../style.css', as_file: true),
+      '/style.css'
+    )
+    t.equal(
+      url.relative_to_absolute('/foo/bar/baz', '../style.css', as_file: true),
       '/foo/style.css'
     )
+    t.equal(
+      url.relative_to_absolute('/articles/article1', 'article2', as_file: true),
+      '/articles/article2'
+    )
   }
 }