no-commit-to-branch: Default to both master and main

README.md

diff --git a/README.md b/README.md
index f66d265..bf36ecf 100644
--- a/README.md
+++ b/README.md
@@ -140,7 +140,7 @@
 #### `no-commit-to-branch`
 Protect specific branches from direct checkins.
   - Use `args: [--branch, staging, --branch, master]` to set the branch.
-    `master` is the default if no branch argument is set.
+    Both `master` and `main` are protected by default if no branch argument is set.
   - `-b` / `--branch` may be specified multiple times to protect multiple
     branches.
   - `-p` / `--pattern` can be used to protect branches that match a supplied regex

pre_commit_hooks/no_commit_to_branch.py

diff --git a/pre_commit_hooks/no_commit_to_branch.py b/pre_commit_hooks/no_commit_to_branch.py
index fb1506f..49ffecf 100644
--- a/pre_commit_hooks/no_commit_to_branch.py
+++ b/pre_commit_hooks/no_commit_to_branch.py
@@ -38,7 +38,7 @@
     )
     args = parser.parse_args(argv)
 
-    protected = frozenset(args.branch or ('master',))
+    protected = frozenset(args.branch or ('master', 'main'))
     patterns = frozenset(args.pattern or ())
     return int(is_on_branch(protected, patterns))
 

tests/no_commit_to_branch_test.py

diff --git a/tests/no_commit_to_branch_test.py b/tests/no_commit_to_branch_test.py
index 72b32e6..610e660 100644
--- a/tests/no_commit_to_branch_test.py
+++ b/tests/no_commit_to_branch_test.py
@@ -67,3 +67,10 @@
         cmd_output('git', 'checkout', head)
         # we're not on a branch!
         assert main(()) == 0
+
+
+@pytest.mark.parametrize('branch_name', ('master', 'main'))
+def test_default_branch_names(temp_git_dir, branch_name):
+    with temp_git_dir.as_cwd():
+        cmd_output('git', 'checkout', '-b', branch_name)
+        assert main(()) == 1