Merge branch 'add-overcommit-support' into 'master'

Adds basic overcommit support

See merge request gitlab-org/gitlab-ce!31840

.gitignore

@@ -81,4 +81,5 @@ package-lock.json
 /junit_*.xml
 /coverage-frontend/
 jsdoc/
-**/tmp/rubocop_cache/**
\ No newline at end of file
+**/tmp/rubocop_cache/**
+.overcommit.yml

.overcommit.yml.example

+# Use this file to configure the Overcommit hooks you wish to use. This will
+# extend the default configuration defined in:
+# https://github.com/sds/overcommit/blob/master/config/default.yml
+#
+# At the topmost level of this YAML file is a key representing type of hook
+# being run (e.g. pre-commit, commit-msg, etc.). Within each type you can
+# customize each hook, such as whether to only run it on certain files (via
+# `include`), whether to only display output if it fails (via `quiet`), etc.
+#
+# For a complete list of hooks, see:
+# https://github.com/sds/overcommit/tree/master/lib/overcommit/hook
+#
+# For a complete list of options that you can use to customize hooks, see:
+# https://github.com/sds/overcommit#configuration
+#
+# Uncomment the following lines to make the configuration take effect.
+
+PreCommit:
+  RuboCop:
+    enabled: true
+#    on_warn: fail # Treat all warnings as failures
+#
+#PostCheckout:
+#  ALL: # Special hook name that customizes all hooks of this type
+#    quiet: true # Change all post-checkout hooks to only display output on failure
+#
+#  IndexTags:
+#    enabled: true # Generate a tags file with `ctags` each time HEAD changes

doc/development/contributing/style_guides.md

@@ -25,8 +25,13 @@
 1. [Python](../python_guide/index.md)
 1. [Shell scripting](../shell_scripting_guide/index.md)
 
+## Checking the style and other issues
+
 This is also the style used by linting tools such as
 [RuboCop](https://github.com/rubocop-hq/rubocop) and [Hound CI](https://houndci.com).
+You can run RuboCop by hand or install a tool like [Overcommit](https://github.com/sds/overcommit) to run it for you.
+Overcommit will automatically run the configured checks (like Rubocop) on every modified file before commit. You can use the example overcommit configuration found in `.overcommit.yml.example` as a quickstart.
+This saves you time as you don't have to wait for the same errors to be detected by the CI.
 
 ---