multiple mutable rev warnings issued on autoupdate

[graingert]

when running pre-commit autoupdate I get 2 warnings per mutable rev, when I expected 0 see #974

~/projects/pytest-cov   pre-commit-autoupdate  pipx run pre-commit autoupdate    
[WARNING] The 'rev' field of repo 'https://github.com/pre-commit/pre-commit-hooks' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[WARNING] The 'rev' field of repo 'https://github.com/timothycrosley/isort' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[WARNING] The 'rev' field of repo 'https://gitlab.com/pycqa/flake8' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[WARNING] The 'rev' field of repo 'https://github.com/pre-commit/pre-commit-hooks' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[WARNING] The 'rev' field of repo 'https://github.com/timothycrosley/isort' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
[WARNING] The 'rev' field of repo 'https://gitlab.com/pycqa/flake8' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
Updating https://github.com/pre-commit/pre-commit-hooks ... updating master -> v4.0.1.
Updating https://github.com/timothycrosley/isort ... updating master -> 5.9.3.
Updating https://gitlab.com/pycqa/flake8 ... updating master -> 3.9.2.

[paulhfischer]

this affects all config validation checks (e.g. also unknown keywords), as the config is validated twice (migration and updating).

This might be fixable by suppressing the logger.warning output if the migration is being called with silent=True, as it is done whilst autoupdating. If this sounds like a reasonable approach to you @asottile, I can change/add this if you want.

(this context should do the trick)

import logging

from contextlib import contextmanager
from typing import Generator

@contextmanager
def suppress_warnings(logger: logging.Logger) -> Generator[None, None, None]:
    old_level = logger.getEffectiveLevel()
    logger.setLevel(logging.ERROR)

    try:
        yield
    finally:
        logger.setLevel(old_level)

[asottile]

since this is cosmetic and harmless I don't want to do any invasive "fixes" for this, but I suspect this line can be removed (as it will need to be removed anyway when list-based configurations are finally removed in 3.x):

pre-commit/pre_commit/commands/migrate_config.py

Lines 43 to 44 in 509e4e2

	# ensure that the configuration is a valid pre-commit configuration
	load_config(config_file)