GitPython: Arbitrary file read via --pathspec-from-file in IndexFile.remove() and Head.checkout()

Summary IndexFile.remove and Head.checkout forward kwargs into git rm and git checkout with no guard. Passing --pathspec-from-file= together with --pathspec-file-nul makes Git treat the whole file as a single NUL-delimited pathspec, and the unmatched-pathspec error quotes it verbatim. GitPython...