Adds support for prepare-commit-msg hooks

Adds a prepare-commit-msg hook stage which allows for hooks which add
dynamic suggested/placeholder text to commit messages that an author can
use as a starting point for writing a commit message

pre_commit/commands/run.py

@@ -190,7 +190,7 @@ def _compute_cols(hooks, verbose):
 def _all_filenames(args):
     if args.origin and args.source:
         return git.get_changed_files(args.origin, args.source)
-    elif args.hook_stage == 'commit-msg':
+    elif args.hook_stage in ['prepare-commit-msg', 'commit-msg']:
         return (args.commit_msg_filename,)
     elif args.files:
         return args.files

pre_commit/constants.py

@@ -21,6 +21,6 @@ LOCAL_REPO_VERSION = '1'
 VERSION = importlib_metadata.version('pre_commit')
 
 # `manual` is not invoked by any installed git hook.  See #719
-STAGES = ('commit', 'commit-msg', 'manual', 'push')
+STAGES = ('commit', 'prepare-commit-msg', 'commit-msg', 'manual', 'push')
 
 DEFAULT = 'default'

pre_commit/main.py

@@ -52,7 +52,9 @@ def _add_config_option(parser):
 
 def _add_hook_type_option(parser):
     parser.add_argument(
-        '-t', '--hook-type', choices=('pre-commit', 'pre-push', 'commit-msg'),
+        '-t', '--hook-type', choices=(
+            'pre-commit', 'pre-push', 'prepare-commit-msg', 'commit-msg',
+        ),
         default='pre-commit',
     )
 

pre_commit/resources/hook-tmpl

@@ -161,6 +161,7 @@ def _pre_push(stdin):
 
 def _opts(stdin):
     fns = {
+        'prepare-commit-msg': lambda _: ('--commit-msg-filename', sys.argv[1]),
         'commit-msg': lambda _: ('--commit-msg-filename', sys.argv[1]),
         'pre-commit': lambda _: (),
         'pre-push': _pre_push,