Feature request: be able to parse git patches

Patches such as https://github.com/ocaml/opam-repository/blob/master/packages/ocaml-base-compiler/ocaml-base-compiler.3.10.0/files/alt-signal-stack.patch fail to parse with the following error:
```
# let diffs = Patch.to_diffs str;;
Exception: Invalid_argument "String.sub / Bytes.sub".
```