subrepo and update asm-differ (#1664)

* git subrepo clone git@github.com:simonlindholm/asm-differ.git tools/asm-differ

subrepo:
  subdir:   "tools/asm-differ"
  merged:   "11eee5916"
upstream:
  origin:   "git@github.com:simonlindholm/asm-differ.git"
  branch:   "main"
  commit:   "11eee5916"
git-subrepo:
  version:  "0.4.6"
  origin:   "https://github.com/ingydotnet/git-subrepo"
  commit:   "110b9eb"

* ln -s ./tools/asm-differ/diff.py diff.py

tools/asm-differ/README.md

@@ -0,0 +1,56 @@
+# asm-differ
+
+Nice differ for assembly code. Currently supports MIPS, PPC, AArch64, ARM32, SH2, SH4, and m68k; should be easy to hack to support other instruction sets.
+
+![](screenshot.png)
+
+## Dependencies
+
+- Python >= 3.6
+- `python3 -m pip install --user colorama watchdog levenshtein cxxfilt` (also `dataclasses` if on 3.6)
+
+## Usage
+
+Create a file `diff_settings.py` in some directory (see the one in this repo for an example). Then from that directory, run
+
+```bash
+/path/to/diff.py [flags] (function|rom addr)
+```
+
+Recommended flags are `-mwo` (automatically run `make` on source file changes, and include symbols in diff). See `--help` for more details.
+
+### Tab completion
+
+[argcomplete](https://kislyuk.github.io/argcomplete/) can be optionally installed (with `python3 -m pip install argcomplete`) to enable tab completion in a bash shell, completing options and symbol names using the linker map. It also requires a bit more setup:
+
+If invoking the script **exactly** as `./diff.py`, the following should be added to the `.bashrc` according to argcomplete's instructions:
+
+```bash
+eval "$(register-python-argcomplete ./diff.py)"
+```
+
+If that doesn't work, run `register-python-argcomplete ./diff.py` in your terminal and copy the output to `.bashrc`.
+
+If setup correctly (don't forget to restart the shell), `complete | grep ./diff.py` should output:
+
+```bash
+complete -o bashdefault -o default -o nospace -F _python_argcomplete ./diff.py
+```
+
+Note for developers or for general troubleshooting: run `export _ARC_DEBUG=` to enable debug output during tab-completion, it may show otherwise silenced errors. Use `unset _ARC_DEBUG` or restart the terminal to disable.
+
+### Contributing
+
+Contributions are very welcome! Some notes on workflow:
+
+`black` is used for code formatting. You can either run `black diff.py` manually, or set up a pre-commit hook:
+```bash
+pip install pre-commit black
+pre-commit install
+```
+
+Type annotations are used for all Python code. `mypy` should pass without any errors.
+
+PRs that skip the above are still welcome, however.
+
+The targeted Python version is 3.6. There are currently no tests.