Print hookid on failure.

pre_commit/commands/run.py

@@ -84,6 +84,7 @@ def _run_single_hook(runner, repository, hook_id, args, write, skips=set()):
     write(color.format_color(pass_fail, print_color, args.color) + '\n')
 
     if (stdout or stderr) and (retcode or args.verbose):
+        write('hookid: {0}\n'.format(hook['id']))
         write('\n')
         for output in (stdout, stderr):
             if output.strip():

tests/commands/install_uninstall_test.py

@@ -157,6 +157,7 @@ FAILING_PRE_COMMIT_RUN = re.compile(
     r'\[INFO\] Once installed this environment will be reused\.\n'
     r'\[INFO\] This may take a few minutes\.\.\.\n'
     r'Failing hook\.+Failed\n'
+    r'hookid: failing_hook\n'
     r'\n'
     r'Fail\n'
     r'foo\n'

tests/commands/run_test.py

@@ -77,7 +77,7 @@ def test_run_all_hooks_failing(
     _test_run(
         repo_with_failing_hook,
         {},
-        ('Failing hook', 'Failed', 'Fail\nfoo.py\n'),
+        ('Failing hook', 'Failed', 'hookid: failing_hook', 'Fail\nfoo.py\n'),
         1,
         True,
     )