Some git-related tests fail when defaultBranch is not master

[guludo]

Bug Report

Description

When git configuration for init.defaultBranch is a value different from master some tests fail.

Reproduce

Steps to reproduce:

1. Set the value of init.defaultBranch to a value other than master:

   $ git config --global init.defaultBranch main

2. Run a git-related test:

      $ python -m tests 'tests/func/test_checkout.py::TestRemoveFilesWhenCheckout::test'
      =============================================================================================== test session starts ================================================================================================
      platform linux -- Python 3.9.7, pytest-6.2.5, py-1.11.0, pluggy-1.0.0 -- /home/guludo/foss/dvc/venv/bin/python
      cachedir: .pytest_cache
      rootdir: /home/guludo/foss/dvc, configfile: pyproject.toml
      plugins: anyio-3.3.4, forked-1.3.0, xdist-2.4.0, mock-3.6.1, lazy-fixture-0.6.3, docker-0.10.3, cov-3.0.0, flaky-3.7.0
      [gw0] linux Python 3.9.7 cwd: /home/guludo/foss/dvc
      [gw1] linux Python 3.9.7 cwd: /home/guludo/foss/dvc
      [gw0] Python 3.9.7 (default, Oct 10 2021, 15:13:22)  -- [GCC 11.1.0]
      [gw1] Python 3.9.7 (default, Oct 10 2021, 15:13:22)  -- [GCC 11.1.0]
      gw0 [1] / gw1 [1]
      scheduling tests via LoadScopeScheduling
   
      tests/func/test_checkout.py::TestRemoveFilesWhenCheckout::test
      [gw0] [100%] FAILED tests/func/test_checkout.py::TestRemoveFilesWhenCheckout::test
      
      (...)   

[skshetry]

Thank you for creating an issue, we are aware of this issue and we have a patch in progress to fix that: #6857. Should be fixed soon. 🤞🏼