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.

pre_commit_hooks/prepare_commit_msg.py

@@ -0,0 +1,123 @@
+from __future__ import annotations
+
+import argparse
+import re
+from typing import Sequence
+
+from jinja2 import Environment
+from jinja2 import PackageLoader
+from jinja2 import select_autoescape
+
+from pre_commit_hooks.util import CalledProcessError
+from pre_commit_hooks.util import cmd_output
+
+
+def get_current_branch() -> str:
+    try:
+        ref_name = cmd_output('git', 'symbolic-ref', '--short', 'HEAD')
+    except CalledProcessError:
+        return ''
+
+    return ref_name.strip()
+
+
+def _configure_args(
+        parser: argparse.ArgumentParser,
+) -> argparse.ArgumentParser:
+    parser.add_argument(
+        '-t', '--template', default='prepare_commit_msg_append.j2',
+        help='Template to use for the commit message.',
+    )
+    parser.add_argument(
+        '-b', '--branch', action='append', default=['main', 'master'],
+        help='Branch to skip, may be specified multiple times.',
+    )
+    parser.add_argument(
+        '-p', '--pattern', action='append', default=['(?<=feature/).*'],
+        help='RegEx Pattern for recognising Ticket Numbers in branch, '
+             'may be specified multiple times.',
+    )
+    parser.add_argument('COMMIT_MSG_FILE', nargs=argparse.REMAINDER)
+
+    return parser
+
+
+def get_jinja_env() -> Environment:
+    return Environment(
+        loader=PackageLoader('pre_commit_hooks'),
+        autoescape=select_autoescape(),
+    )
+
+
+def get_rendered_template(
+        jinja: Environment,
+        template_file: str,
+        variables: dict[str, str],
+) -> str:
+    template = jinja.get_template(template_file)
+    return template.render(variables)
+
+
+def update_commit_file(
+        jinja: Environment,
+        commit_msg_file: str,
+        template: str,
+        ticket: str,
+) -> int:
+    try:
+        with open(commit_msg_file) as f:
+            data = f.readlines()
+
+        data_as_str = ''.join([item for item in data])
+        # if message already contain ticket number means
+        # it is under git commit --amend or rebase or alike
+        # where message was already set in the past
+        if ticket in data_as_str:
+            return 0
+
+        variables = {
+            'ticket': ticket,
+            'content': data_as_str,
+        }
+
+        content = get_rendered_template(
+            jinja=jinja,
+            template_file=template,
+            variables=variables,
+        )
+        with open(commit_msg_file, 'w') as f:
+            f.write(content)
+
+        return 0
+    except OSError as err:
+        print(f'OS error: {err}')
+        return 1
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = _configure_args(argparse.ArgumentParser())
+    args = parser.parse_args(argv)
+
+    current = get_current_branch()
+    branches = frozenset(args.branch)
+    if current in branches:
+        # checked black listed branches
+        return 0
+
+    patterns = frozenset(args.pattern)
+    matches = [
+        match.group(0)
+        for match in (re.search(pattern, current) for pattern in patterns)
+        if match
+    ]
+    if len(matches) == 0:
+        # checked white listed branches
+        return 0
+
+    jinja = get_jinja_env()
+    commit_file = args.COMMIT_MSG_FILE[0]
+    return update_commit_file(jinja, commit_file, args.template, matches[0])
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())

pre_commit_hooks/templates/prepare_commit_msg_append.j2

@@ -0,0 +1,3 @@
+{{ content }}
+
+Relates: {{ ticket }}

pre_commit_hooks/templates/prepare_commit_msg_prepend.j2

@@ -0,0 +1 @@
+[{{ ticket }}] {{ content }}