Split tox "lint" env into three envs, all safe

It makes sense for ruff linting and ruff autoformatting to be
easily runnable individually and to have their results be shown
separately. Splitting them out in tox.ini also makes it so tox can
do the other tests corresponding to those in lint.yml on CI in an
environment that requries no custom behavior from pre-commit other
than skipping the ruff checks.

The ruff linting ("ruff") and ruff format checking ("format") tox
environments specify ruff as a dependency and call it directly
rather than through pre-commit, invoking it in such a way that it
does not attempt to modify any files in the working tree.

See b059cd5 (#1868) for context. All three of these tox envs that
"lint" has been split into are listed in the env_list and thus run
automatically when tox is run with no arguments, since no tox envs
unexpectedly (or at all) modify files in the working tree anymore.

One limitation of the current approach is that new pre-commit hooks
configured in .pre-commit-config.yml will automatically be run as
part of the "misc" tox environment, which means that new unexpected
mutating operatons could be added if the impact on tox is not
considered. The benefit of having it work this way is that most
hooks that are likely to be added to GitPython would not modify
files and would be wanted as part of "misc". But this may benefit
from further refinement.

Author: EliahKagan

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 requires = tox>=4
-env_list = py{37,38,39,310,311,312}, mypy, html
+env_list = py{37,38,39,310,311,312}, ruff, format, mypy, html, misc
 
 [testenv]
 description = Run unit tests
@@ -9,10 +9,17 @@ extras = test
 pass_env = SSH_*
 commands = pytest --color=yes {posargs}
 
-[testenv:lint]
-description = Lint via pre-commit
+[testenv:ruff]
+description = Lint with Ruff
 base_python = py{39,310,311,312,38,37}
-commands = pre-commit run --all-files
+deps = ruff
+commands = ruff check .
+
+[testenv:format]
+description = Check formatting with Ruff
+base_python = py{39,310,311,312,38,37}
+deps = ruff
+commands = ruff format --check .
 
 [testenv:mypy]
 description = Typecheck with mypy
@@ -28,3 +35,10 @@ allowlist_externals = make
 commands =
     make BUILDDIR={env_tmp_dir}/doc/build -C doc clean
     make BUILDDIR={env_tmp_dir}/doc/build -C doc html
+
+[testenv:misc]
+description = Run other checks via pre-commit
+base_python = py{39,310,311,312,38,37}
+set_env =
+    SKIP = ruff-format,ruff
+commands = pre-commit run --all-files