Strip extra slashes via `Jekyll.sanitized_path` (#7182)

Merge pull request 7182

lib/jekyll.rb

@@ -176,6 +176,10 @@ module Jekyll
 
       return clean_path if clean_path.eql?(base_directory)
 
+      # remove any remaining extra leading slashes not stripped away by calling
+      # `File.expand_path` above.
+      clean_path.squeeze!("/")
+
       if clean_path.start_with?(base_directory.sub(%r!\z!, "/"))
         clean_path
       else

test/test_path_sanitization.rb

@@ -31,6 +31,13 @@ class TestPathSanitization < JekyllUnitTest
                  Jekyll.sanitized_path(source_dir, "f./../../../../../../files/hi.txt")
   end
 
+  should "strip extra slashes in questionable path" do
+    subdir = "/files/"
+    file_path = "/hi.txt"
+    assert_equal source_dir("files", "hi.txt"),
+                 Jekyll.sanitized_path(source_dir, "/#{subdir}/#{file_path}")
+  end
+
   if Jekyll::Utils::Platforms.really_windows?
     context "on Windows with absolute path" do
       setup do