Use the legacy Gemfile/Gemfile.lock names in tooling/overcommit

This is because when multiple versions of Bundler are installed (e.g.
1.17.3 and 2.1.4), Bundler will choose the 2.x version based on the
`gems.rb` / `gems.locked` file names but it chooses the 1.17.x version
if the we use the legacy `Gemfile` / `Gemfile.lock` file names.

For now, we'll use the legacy file names to be consistent with the main
`gitlab`'s `Gemfile.lock` and to avoid unexpected changes in the
working tree.
Signed-off-by: Rémy Coutable <remy@rymai.me>

.overcommit.yml.example

@@ -15,8 +15,8 @@
 #
 # Uncomment the following lines to make the configuration take effect.
 
-# Make sure to run `cd tooling/overcommit && make && cd -`
-gemfile: 'tooling/overcommit/gems.rb'
+# Make sure to run `make -C tooling/overcommit`
+gemfile: 'tooling/overcommit/Gemfile'
 
 PostCheckout:
   BundleInstall:

doc/development/contributing/style_guides.md

@@ -17,7 +17,7 @@ static analysis offenses before committing locally.
 In your GitLab source directory run:
 
 ```shell
-cd tooling/overcommit && make && cd -
+make -C tooling/overcommit
 ```
 
 Then before a commit is created, Overcommit will automatically check for

tooling/overcommit/gems.rb → tooling/overcommit/Gemfile

 # frozen_string_literal: true
 
-# Make sure to run `bundle install --gemfile=tooling/overcommit/gems.rb` when you update this file.
+# Make sure to run `make` (in this directory) when you update this file.
 source 'https://rubygems.org'
 
 gem 'overcommit'