Add prepare-commit-msg hook

Added new hook for templating prepare-commit-msg hook. Added 2
templates to use keepgin `prepare_commit_msg_append.j2`as default.

The idea is that developers could create their own templates accordingly
to teams commit conventions and override them in the hook arguments when
configuring it.

tests/prepare_commit_msg_test.py

@@ -0,0 +1,95 @@
+from __future__ import annotations
+
+import pytest
+
+from pre_commit_hooks.prepare_commit_msg import get_current_branch
+from pre_commit_hooks.prepare_commit_msg import get_jinja_env
+from pre_commit_hooks.prepare_commit_msg import main
+from pre_commit_hooks.prepare_commit_msg import update_commit_file
+from pre_commit_hooks.util import cmd_output
+
+
+def test_current_branch(temp_git_dir):
+    with temp_git_dir.as_cwd():
+        cmd_output('git', 'checkout', '-b', 'feature')
+        assert get_current_branch() == 'feature'
+
+        cmd_output('git', 'checkout', '-b', 'feature/branch')
+        assert get_current_branch() == 'feature/branch'
+
+
+# Input, expected value, branch, template
+TESTS = (
+    (
+        b'',
+        b'[TT-01] ',
+        'feature/TT-01',
+        'prepare_commit_msg_prepend.j2',
+    ),
+    (
+        b'[TT-02] Some message',
+        b'[TT-02] Some message',
+        'feature/TT-02',
+        'prepare_commit_msg_prepend.j2',
+    ),
+    (
+        b'Initial message',
+        b'[TT-03] Initial message',
+        'feature/TT-03',
+        'prepare_commit_msg_prepend.j2',
+    ),
+    (
+        b'',
+        b'\n\nRelates: AA-01',
+        'feature/AA-01',
+        'prepare_commit_msg_append.j2',
+    ),
+    (
+        b'Initial message',
+        b'Initial message\n\nRelates: AA-02',
+        'feature/AA-02',
+        'prepare_commit_msg_append.j2',
+    ),
+)
+
+
+@pytest.mark.parametrize(
+    ('input_s', 'expected_val', 'branch_name', 'template'),
+    TESTS,
+)
+def test_update_commit_file(
+        input_s, expected_val, branch_name, template,
+        temp_git_dir,
+):
+    with temp_git_dir.as_cwd():
+        path = temp_git_dir.join('COMMIT_EDITMSG')
+        path.write_binary(input_s)
+        ticket = branch_name.split('/')[1]
+        jinja = get_jinja_env()
+        update_commit_file(jinja, path, template, ticket)
+
+        assert path.read_binary() == expected_val
+
+
+@pytest.mark.parametrize(
+    ('input_s', 'expected_val', 'branch_name', 'template'),
+    TESTS,
+)
+def test_main(
+        input_s, expected_val, branch_name, template,
+        temp_git_dir,
+):
+    with temp_git_dir.as_cwd():
+        path = temp_git_dir.join('file.txt')
+        path.write_binary(input_s)
+        assert path.read_binary() == input_s
+
+        cmd_output('git', 'checkout', '-b', branch_name)
+        assert main(
+            argv=[
+                '-t', template,
+                '-p', '(?<=feature/).*',
+                str(path),
+            ],
+        ) == 0
+        assert path.read_binary() == expected_val