Pre-commit

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- Recover from invalid python virtualenvs

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- Work around a windows-specific virtualenv bug pypa/virtualenv1062
This failure mode was introduced in 0.15.1

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- Use a more intelligent default language version for python

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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`