Add pre-commit try-repo

`try-repo` is useful for:
- Trying out a remote hook repository without needing to configure it.
- Testing a hook repository while developing it.

testing/fixtures.py

@@ -10,6 +10,7 @@ from aspy.yaml import ordered_dump
 from aspy.yaml import ordered_load
 
 import pre_commit.constants as C
+from pre_commit import git
 from pre_commit.clientlib import CONFIG_SCHEMA
 from pre_commit.clientlib import load_manifest
 from pre_commit.schema import apply_defaults
@@ -17,7 +18,6 @@ from pre_commit.schema import validate
 from pre_commit.util import cmd_output
 from pre_commit.util import copy_tree_to_path
 from pre_commit.util import cwd
-from testing.util import get_head_sha
 from testing.util import get_resource_path
 
 
@@ -84,7 +84,7 @@ def make_config_from_repo(repo_path, sha=None, hooks=None, check=True):
     manifest = load_manifest(os.path.join(repo_path, C.MANIFEST_FILE))
     config = OrderedDict((
         ('repo', 'file://{}'.format(repo_path)),
-        ('sha', sha or get_head_sha(repo_path)),
+        ('sha', sha or git.head_sha(repo_path)),
         (
             'hooks',
             hooks or [OrderedDict((('id', hook['id']),)) for hook in manifest],

testing/util.py

@@ -8,7 +8,7 @@ from pre_commit import parse_shebang
 from pre_commit.languages.docker import docker_is_running
 from pre_commit.languages.pcre import GREP
 from pre_commit.util import cmd_output
-from pre_commit.util import cwd
+from testing.auto_namedtuple import auto_namedtuple
 
 
 TESTING_DIR = os.path.abspath(os.path.dirname(__file__))
@@ -18,11 +18,6 @@ def get_resource_path(path):
     return os.path.join(TESTING_DIR, 'resources', path)
 
 
-def get_head_sha(dir):
-    with cwd(dir):
-        return cmd_output('git', 'rev-parse', 'HEAD')[1].strip()
-
-
 def cmd_output_mocked_pre_commit_home(*args, **kwargs):
     # keyword-only argument
     tempdir_factory = kwargs.pop('tempdir_factory')
@@ -72,3 +67,31 @@ xfailif_no_symlink = pytest.mark.xfail(
     not hasattr(os, 'symlink'),
     reason='Symlink is not supported on this platform',
 )
+
+
+def run_opts(
+        all_files=False,
+        files=(),
+        color=False,
+        verbose=False,
+        hook=None,
+        origin='',
+        source='',
+        hook_stage='commit',
+        show_diff_on_failure=False,
+        commit_msg_filename='',
+):
+    # These are mutually exclusive
+    assert not (all_files and files)
+    return auto_namedtuple(
+        all_files=all_files,
+        files=files,
+        color=color,
+        verbose=verbose,
+        hook=hook,
+        origin=origin,
+        source=source,
+        hook_stage=hook_stage,
+        show_diff_on_failure=show_diff_on_failure,
+        commit_msg_filename=commit_msg_filename,
+    )