Introduce a Gemfile dedicated to Overcommit to avoid conflicts

Running Overcommit outside of the context of a Gemfile can lead to
conflicts when requiring gems. Using a dedicated Gemfile instead of the
default one avoids loading all the app's gems before running Overcommit.
Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitignore

@@ -83,6 +83,7 @@ package-lock.json
 jsdoc/
 **/tmp/rubocop_cache/**
 .overcommit.yml
+.overcommit.yml.backup
 .projections.json
 /qa/.rakeTasks
 webpack-dev-server.json

.overcommit.yml.example

@@ -15,6 +15,13 @@
 #
 # Uncomment the following lines to make the configuration take effect.
 
+# Make sure to run `cd tooling/overcommit && make && cd -`
+gemfile: 'tooling/overcommit/gems.rb'
+
+PostCheckout:
+  BundleInstall:
+    enabled: true
+
 PreCommit:
   AuthorName:
     enabled: false
@@ -35,6 +42,19 @@ PreCommit:
 #    on_warn: fail # Treat all warnings as failures
   ScssLint:
     enabled: true
+
+CommitMsg:
+  TextWidth:
+    enabled: true
+    min_subject_width: 8 # three 2-letter words with 2 spaces
+    max_subject_width: 72
+    quiet: false
+
+  EmptyMessage:
+    enabled: true
+    required: true
+    description: 'Checking for empty commit message'
+
 #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

doc/development/contributing/style_guides.md

@@ -30,7 +30,9 @@
 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.
+
+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.
 
 ---

tooling/overcommit/Makefile

+.PHONY: install
+
+install:
+	if [ -e ../../.overcommit.yml ]; then cp -f ../../.overcommit.yml ../../.overcommit.yml.backup; fi
+	cp ../../.overcommit.yml.example ../../.overcommit.yml
+	bundle install
+	cd ../../ && overcommit -i

tooling/overcommit/gems.locked

+GEM
+  remote: https://rubygems.org/
+  specs:
+    ast (2.4.0)
+    childprocess (3.0.0)
+    ffi (1.12.1)
+    gitlab-styles (3.1.0)
+      rubocop (~> 0.74.0)
+      rubocop-gitlab-security (~> 0.1.0)
+      rubocop-performance (~> 1.4.1)
+      rubocop-rails (~> 2.0)
+      rubocop-rspec (~> 1.36)
+    haml (5.1.2)
+      temple (>= 0.8.0)
+      tilt
+    haml_lint (0.34.1)
+      haml (>= 4.0, < 5.2)
+      rainbow
+      rubocop (>= 0.50.0)
+      sysexits (~> 1.1)
+    iniparse (1.4.4)
+    jaro_winkler (1.5.4)
+    overcommit (0.52.1)
+      childprocess (>= 0.6.3, < 4)
+      iniparse (~> 1.4)
+    parallel (1.19.1)
+    parser (2.7.0.2)
+      ast (~> 2.4.0)
+    rack (2.1.1)
+    rainbow (3.0.0)
+    rake (12.3.3)
+    rb-fsevent (0.10.3)
+    rb-inotify (0.10.1)
+      ffi (~> 1.0)
+    rubocop (0.74.0)
+      jaro_winkler (~> 1.5.1)
+      parallel (~> 1.10)
+      parser (>= 2.6)
+      rainbow (>= 2.2.2, < 4.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 1.4.0, < 1.7)
+    rubocop-gitlab-security (0.1.1)
+      rubocop (>= 0.51)
+    rubocop-performance (1.4.1)
+      rubocop (>= 0.71.0)
+    rubocop-rails (2.4.1)
+      rack (>= 1.1)
+      rubocop (>= 0.72.0)
+    rubocop-rspec (1.37.1)
+      rubocop (>= 0.68.1)
+    ruby-progressbar (1.10.1)
+    sass (3.5.7)
+      sass-listen (~> 4.0.0)
+    sass-listen (4.0.0)
+      rb-fsevent (~> 0.9, >= 0.9.4)
+      rb-inotify (~> 0.9, >= 0.9.7)
+    scss_lint (0.56.0)
+      rake (>= 0.9, < 13)
+      sass (~> 3.5.3)
+    sysexits (1.2.0)
+    temple (0.8.2)
+    tilt (2.0.10)
+    unicode-display_width (1.6.1)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  gitlab-styles (~> 3.1.0)
+  haml_lint (~> 0.34.0)
+  overcommit
+  scss_lint (~> 0.56.0)
+
+BUNDLED WITH
+   2.1.2

tooling/overcommit/gems.rb

+# frozen_string_literal: true
+
+# Make sure to run `bundle install --gemfile=tooling/overcommit/gems.rb` when you update this file.
+source 'https://rubygems.org'
+
+gem 'overcommit'
+gem 'gitlab-styles', '~> 3.1.0', require: false
+gem 'scss_lint', '~> 0.56.0', require: false
+gem 'haml_lint', '~> 0.34.0', require: false