HackStudio: Migrate git-diff indicators to TextEditor API

As part of this, the CodeDocument now keeps track of the kind of
difference for each line. Previously, we iterated every hunk every time
the editor was painted, but now we do that once whenever the diff
changes, and then save the type of difference for each line.

Userland/DevTools/HackStudio/CodeDocument.cpp

@@ -32,4 +32,14 @@ CodeDocument::CodeDocument(Client* client)
 {
 }
 
+CodeDocument::DiffType CodeDocument::line_difference(size_t line) const
+{
+    return m_line_differences[line];
+}
+
+void CodeDocument::set_line_differences(Badge<HackStudio::Editor>, Vector<HackStudio::CodeDocument::DiffType> line_differences)
+{
+    m_line_differences = move(line_differences);
+}
+
 }