Add .gitattributes file

See wp-cli/wp-cli#5070

Update file(s) from wp-cli/.github

Merge pull request wp-cli#194 from wp-cli/refine-feature-files

Merge pull request wp-cli#189 from wp-cli/no-lock-file

Merge pull request wp-cli#185 from wp-cli/revert-181-94-refactor-conf…

…ig-command

Merge pull request wp-cli#181 from UmeshSingla/94-refactor-config-com…

…mand

Fix incorrect message shuffling salt in PHP 5.6 (wp-cli#176)

* Fix duplicate warning

* Add feature test for duplicate warning when shuffling salts

Merge pull request wp-cli#171 from wojtekn/add/mysql-sock-support

Merge pull request wp-cli#166 from wp-cli/no-time-to-die

Merge pull request wp-cli#164 from Ancocodet/fail-on-empty-table-prefix