bbatsov/rubocop

### New features

* [#5980](https://github.com/rubocop/rubocop/issues/5980): Add `--safe` and `--safe-auto-correct` options. ([@Darhazer][])
* [#4156](https://github.com/rubocop/rubocop/issues/4156): Add command line option `--auto-gen-only-exclude`. ([@Ana06][], [@jonas054][])
* [#6386](https://github.com/rubocop/rubocop/pull/6386): Add `VersionAdded` meta data to config/default.yml when running `rake new_cop`. ([@koic][])
* [#6395](https://github.com/rubocop/rubocop/pull/6395): Permit to specify TargetRubyVersion 2.6. ([@koic][])
* [#6392](https://github.com/rubocop/rubocop/pull/6392): Add `Whitelist` config to `Rails/SkipsModelValidations` rule. ([@DiscoStarslayer][])

### Bug fixes

* [#6330](https://github.com/rubocop/rubocop/issues/6330): Fix an error for `Rails/ReversibleMigration` when using variable assignment. ([@koic][], [@scottmatthewman][])
* [#6331](https://github.com/rubocop/rubocop/issues/6331): Fix a false positive for `Style/RedundantFreeze` and a false negative for `Style/MutableConstant` when assigning a regexp object to a constant. ([@koic][])
* [#6334](https://github.com/rubocop/rubocop/pull/6334): Fix a false negative for `Style/RedundantFreeze` when assigning a range object to a constant. ([@koic][])
* [#5538](https://github.com/rubocop/rubocop/issues/5538): Fix false negatives in modifier cops when line length cop is disabled. ([@drenmi][])
* [#6340](https://github.com/rubocop/rubocop/pull/6340): Fix an error for `Rails/ReversibleMigration` when block argument is empty. ([@koic][])
* [#6274](https://github.com/rubocop/rubocop/issues/6274): Fix "[Corrected]" message being displayed even when nothing has been corrected. ([@jekuta][])
* [#6380](https://github.com/rubocop/rubocop/pull/6380): Allow use of a hyphen-separated frozen string literal in Emacs style magic comment. ([@y-yagi][])
* Fix and improve `LineLength` cop for tab-indented code. ([@AlexWayfer][])

### Changes

* [#3727](https://github.com/rubocop/rubocop/issues/3727): Enforce single spaces for `key` option in `Layout/AlignHash` cop. ([@albaer][])
* [#6321](https://github.com/rubocop/rubocop/pull/6321): Fix run of RuboCop when cache directory is not writable. ([@Kevinrob][])

[@Darhazer]: https://github.com/Darhazer
[@Ana06]: https://github.com/Ana06
[@jonas054]: https://github.com/jonas054
[@koic]: https://github.com/koic
[@DiscoStarslayer]: https://github.com/DiscoStarslayer
[@scottmatthewman]: https://github.com/scottmatthewman
[@drenmi]: https://github.com/drenmi
[@jekuta]: https://github.com/jekuta
[@y-yagi]: https://github.com/y-yagi
[@AlexWayfer]: https://github.com/AlexWayfer
[@albaer]: https://github.com/albaer
[@Kevinrob]: https://github.com/Kevinrob