Pre-commit

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- Add `types` and `exclude_types` for filtering files. These options take
an array of "tags" identified for each file. The tags are sourced from
[identify](https://github.com/chriskuehl/identify). One can list the tags
for a file by running `identify-cli filename`.
- `files` is now optional (defaulting to `''`)
- `always_run` + missing `files` also defaults to `files: ''` (previously it
defaulted to `'^$'` (this reverses e150921c).

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- Expose `--origin` and `--source` as `PRE_COMMIT_ORIGIN` and
`PRE_COMMIT_SOURCE` environment variables when running as `pre-push`.

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- Use `--no-ext-diff` when running `git diff`

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- Don't crash when `always_run` is `True` and `files` is not provided.
- Set `VIRTUALENV_NO_DOWNLOAD` when making python virtualenvs.

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- Add a `pre-commit sample-config` command
- Enable ansi color escapes on modern windows
- `autoupdate` now defaults to `--tags-only`, use `--bleeding-edge` for the
old behavior
- Add support for `log_file` in hook configuration to tee hook output to a
file for CI consumption, etc.
- Fix crash with unicode commit messages during merges in python 2.
- Add a `pass_filenames` option to allow disabling automatic filename
positional arguments to hooks.

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- Fix regression in 0.13.5: allow `always_run` and `files` together despite
doing nothing.