HackStudio: Use String instead of LexicalPath

LexicalPath is a 'heavier' object than a String that is mainly used for
path parsing and validation, we don't actually need any of that in
GitRepo and its related files, so let's move to String :^)

I've also done some east-const conversion in the files that I was
editing for the string change.

Userland/DevTools/HackStudio/Git/GitFilesView.h

@@ -6,14 +6,13 @@
 
 #pragma once
 
-#include <AK/LexicalPath.h>
 #include <LibGUI/ListView.h>
 #include <LibGfx/Bitmap.h>
 
 namespace HackStudio {
 
 // A "GitFileAction" is either the staging or the unstaging of a file.
-using GitFileActionCallback = Function<void(const LexicalPath& file)>;
+using GitFileActionCallback = Function<void(String const& file)>;
 
 class GitFilesView : public GUI::ListView {
     C_OBJECT(GitFilesView)