fix: catch missing arg if using {prepare-}commit-msg stage

If using the prepare-commit-msg and commit-msg stages specifically (such
    as with the try-repo command), the `--commit-msg-filename` arg must be
provided.

[fixes pre-commit#1336]

chore: improve error message for hook stage check

pre_commit/commands/run.py

@@ -306,6 +306,15 @@ def run(
             f'`git add {config_file}` to fix this.',
         )
         return 1
+    if (
+            args.hook_stage in {'prepare-commit-msg', 'commit-msg'} and
+            not args.commit_msg_filename
+    ):
+        logger.error(
+            f'`--commit-msg-filename` is required for '
+            f'`--hook-stage {args.hook_stage}`',
+        )
+        return 1
 
     # Expose origin / source as environment variables for hooks to consume
     if args.origin and args.source:

tests/commands/run_test.py

@@ -663,12 +663,7 @@ def test_stages(cap_out, store, repo_with_passing_hook):
                 'language': 'pygrep',
                 'stages': [stage],
             }
-            for i, stage in enumerate(
-                (
-                    'commit', 'push', 'manual', 'prepare-commit-msg',
-                    'commit-msg',
-                ), 1,
-            )
+            for i, stage in enumerate(('commit', 'push', 'manual'), 1)
         ],
     }
     add_config_to_repo(repo_with_passing_hook, config)
@@ -686,8 +681,6 @@ def _run_for_stage(stage):
     assert _run_for_stage('commit').startswith(b'hook 1...')
     assert _run_for_stage('push').startswith(b'hook 2...')
     assert _run_for_stage('manual').startswith(b'hook 3...')
-    assert _run_for_stage('prepare-commit-msg').startswith(b'hook 4...')
-    assert _run_for_stage('commit-msg').startswith(b'hook 5...')
 
 
 def test_commit_msg_hook(cap_out, store, commit_msg_repo):
@@ -819,6 +812,16 @@ def test_error_with_unstaged_config(cap_out, store, modified_config_repo):
     assert ret == 1
 
 
+def test_commit_msg_missing_filename(cap_out, store, repo_with_passing_hook):
+    args = run_opts(hook_stage='commit-msg')
+    ret, printed = _do_run(cap_out, store, repo_with_passing_hook, args)
+    assert ret == 1
+    assert printed == (
+        b'[ERROR] `--commit-msg-filename` is required for '
+        b'`--hook-stage commit-msg`\n'
+    )
+
+
 @pytest.mark.parametrize(
     'opts', (run_opts(all_files=True), run_opts(files=[C.CONFIG_FILE])),
 )