Fix delete-merged-branch to avoid deleting master branch

[bric3]

This should fix #1132

git-delete-merged-branch already checks if the branch is the default branch via git_extra_default_branch helper function, but the implementation only checked init.defaultBranch.

This new implementation goes further by looking in this order:

1. Check explicit config git-extras.default-branch
2. Remote HEAD auto-detection
3. Check git's default branch config init.defaultBranch
4. Common branch detection - Checks if main, master, trunk, default, development, stable, release, prod, production exist locally
5. Fallback to main as last resort

Removed default as a default branch as it was breaking this test, so I preferred to back out.

git-extras/tests/test_git_archive_file.py

Lines 55 to 62 in 6f4cf0c

	@pytest.mark.parametrize("named_temp_repo", ["backslash\\dir"], indirect=True)
	def test_archive_file_on_dirname_has_backslash(self, named_temp_repo):
	named_temp_repo.invoke_installed_extras_command("archive-file")
	git = named_temp_repo.get_repo_git()
	filename = "{0}.{1}.{2}.zip".format(
	"backslash-dir", git.describe("--always", "--long"), "default"
	)
	assert filename in os.listdir()

[hyperupcall]

Thanks! Will review this in a few days - feel free to ping me if I forget!