fixup! Fix serving files that clash with directories

Replace the array spltting and while looping system with a simpler(?)
index check for the last `/`.

You should be able to autosquash out this commit, since it is a fixup
commit.

lib/jekyll/commands/serve/servlet.rb

@@ -25,7 +25,6 @@ def search_file(req, res, basename)
           super || super(req, res, "#{basename}.html")
         end
 
-        # rubocop:disable Lint/AssignmentInCondition
         def search_index_file(req, res)
           # /file/index.html -> /file.html
 
@@ -37,33 +36,21 @@ def search_index_file(req, res)
             # Ok, I guess that didn't work, I guess there's no basename/index.html
             # Let's look for basename.html instead...
 
-            # Keep a backup of res.filename in case we need to revert our changes to it
-            old_filename = res.filename
-
-            # We need to calculate the basename and remove it from the path (res.filename)
-            # so we'll turn res.filename into an array of path elements then pop off the
-            # basename.
-            #
-            # We use a while loop just in case res.filename has trailing slashes.
-            #
-            # Once we have popped off the basename, we can join up what's left and use it
-            # as the new res.filename.
-            path_arr = res.filename.scan(%r!/[^/]*!)
-            while basename = path_arr.pop
-              break unless basename == "/"
-            end
-            res.filename = path_arr.join
+            # Split the basename and split it from the path (res.filename)
+            # Regex matches a / that isn't followed by another / or EOL
+            index = res.filename.rindex %r!/(?!/|$)!
+            basename = res.filename[index..-1]
+            res.filename = res.filename[0, index]
 
             # Try and find a file named dirname.html in the parent directory.
             file = search_file(req, res, basename + ".html")
 
-            # If we didn't find a file, revert our changes to res.filename .
-            res.filename = old_filename unless file
+            # If we didn't find a file, revert our changes to res.filename.
+            res.filename << basename unless file
           end
 
           return file
         end
-        # rubocop:enable Lint/AssignmentInCondition
 
         # rubocop:disable Style/MethodName
         def do_GET(req, res)