test: fix test failure in case of different default branch (#6857)

* test: fix test failure in case of different default branch

When the git config has set global `init.defaultBranch` different than master,
a few test cases were failing.
Currently, the test case is assuming the default branch to be set as
master.

This commit will introduce **initial_branch** option to git init call
which will ensure *master* branch is used as default for out test cases.

Set default git branch to main for test CI

Since main is going to be new default branch, it will let the test case
handle the scenario where default branch is set other than master.

Introduce isolation for git configuration

Co-authored-by: Saugat Pachhai (सौगात) <[email protected]>

tests/func/test_import.py

@@ -4,6 +4,7 @@ from unittest.mock import patch
 
 import pytest
 from funcy import first
+from scmrepo.git import Git
 
 from dvc.config import NoRemoteError
 from dvc.dvcfile import Dvcfile
@@ -268,9 +269,7 @@ def test_pull_wildcard_imported_directory_stage(tmp_dir, dvc, erepo_dir):
 def test_push_wildcard_from_bare_git_repo(
     tmp_dir, make_tmp_dir, erepo_dir, local_cloud
 ):
-    import git
-
-    git.Repo.init(os.fspath(tmp_dir), bare=True)
+    Git.init(tmp_dir.fs_path, bare=True).close()
 
     erepo_dir.add_remote(config=local_cloud.config)
     with erepo_dir.chdir():
@@ -381,9 +380,7 @@ def test_pull_no_rev_lock(erepo_dir, tmp_dir, dvc):
 def test_import_from_bare_git_repo(
     tmp_dir, make_tmp_dir, erepo_dir, local_cloud
 ):
-    import git
-
-    git.Repo.init(os.fspath(tmp_dir), bare=True)
+    Git.init(tmp_dir.fs_path, bare=True).close()
 
     erepo_dir.add_remote(config=local_cloud.config)
     with erepo_dir.chdir():