Add a black check to pre-commit

- Add the psf/black-pre-commit-mirror pre-commit hook but have it
  just check and report violations rather than fixing them
  automatically (to avoid inconsistency with all the other hooks,
  and also so that the linting instructions and CI lint workflow
  can remain the same and automatically do the black check).

- Remove the "black" environment from tox.ini, since it is now part
  of the linting done in the "lint" environment. (But README.md
  remains the same, as it documented actually auto-formatting with
  black, which is still done the same way.)

- Add missing "exclude" keys for the shellcheck and black
  pre-commit hooks to explicitly avoid traversing into the
  submodules.

Author: EliahKagan

Diff for: .pre-commit-config.yaml

@@ -1,4 +1,11 @@
 repos:
+  - repo: https://github.com/psf/black-pre-commit-mirror
+    rev: 23.9.1
+    hooks:
+      - id: black
+        args: [--check, --diff]
+        exclude: ^git/ext/
+
   - repo: https://github.com/PyCQA/flake8
     rev: 6.1.0
     hooks:
@@ -14,6 +21,7 @@ repos:
     hooks:
       - id: shellcheck
         args: [--color]
+        exclude: ^git/ext/
 
   - repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v4.4.0

Diff for: tox.ini

@@ -1,6 +1,6 @@
 [tox]
 requires = tox>=4
-env_list = py{37,38,39,310,311,312}, lint, mypy, black
+env_list = py{37,38,39,310,311,312}, lint, mypy
 
 [testenv]
 description = Run unit tests
@@ -20,11 +20,6 @@ base_python = py39
 commands = mypy -p git
 ignore_outcome = true
 
-[testenv:black]
-description = Check style with black
-base_python = py39
-commands = black --check --diff .
-
 # Run "tox -e html" for this. It is deliberately excluded from env_list, as
 # unlike the other environments, this one writes outside the .tox/ directory.
 [testenv:html]