Add Git LFS pre-push hook

Git LFS installs pre-push hook which:
- checks that git-lfs utility is on PATH, and if no, warns that
Git LFS hook should probably be removed
- calls it by executing `git lfs pre-push <remote_name> <remote_url>`

The introduced hook is a Ruby port of the same algorithm
modulo hook disabling note in the warning message.

README.md

@@ -549,6 +549,7 @@ but before any objects have been transferred. If a hook fails, the push is
 aborted.
 
 * [Brakeman](lib/overcommit/hook/pre_push/brakeman.rb)
+* [GitLfs](lib/overcommit/hook/pre_push/git_lfs.rb)
 * [Minitest](lib/overcommit/hook/pre_push/minitest.rb)
 * [ProtectedBranches](lib/overcommit/hook/pre_push/protected_branches.rb)
 * [Pytest](lib/overcommit/hook/pre_push/pytest.rb)

config/default.yml

@@ -945,6 +945,10 @@ PrePush:
     flags: ['--exit-on-warn', '--quiet', '--summary']
     install_command: 'gem install brakeman'
 
+  GitLfs:
+    enabled: false
+    description: 'Upload files tracked by Git LFS'
+
 # Hooks that run during `git rebase`, before any commits are rebased.
 # If a hook fails, the rebase is aborted.
 PreRebase:

lib/overcommit/hook/pre_push/git_lfs.rb

@@ -0,0 +1,21 @@
+module Overcommit::Hook::PrePush
+  # Invokes Git LFS command that uploads files tracked by Git LFS to the LFS storage
+  #
+  # @see https://git-lfs.github.com/
+  class GitLfs < Base
+    def run
+      result = execute(['command', '-v', 'git-lfs'])
+      unless result.success?
+        return :warn, 'This repository is configured for Git LFS but \'git-lfs\' ' \
+        'was not found on your path.\nIf you no longer wish to use Git LFS, ' \
+        'disable this hook by removing or setting \'enabled: false\' for GitLFS ' \
+        'hook in your .overcommit.yml file'
+      end
+
+      result = execute(['git', 'lfs', 'pre-push', remote_name, remote_url])
+      return :fail, result.stderr unless result.success?
+
+      :pass
+    end
+  end
+end

spec/overcommit/hook/pre_push/git_lfs_spec.rb

@@ -0,0 +1,39 @@
+require 'spec_helper'
+
+describe Overcommit::Hook::PrePush::GitLfs do
+  let(:config)  { Overcommit::ConfigurationLoader.default_configuration }
+  let(:context) { double('context', remote_name: 'remote_name', remote_url: 'remote_url') }
+  subject { described_class.new(config, context) }
+
+  let(:result) { double('result') }
+
+  before do
+    subject.stub(:execute).and_return(result)
+  end
+
+  context 'when git-lfs is not on path' do
+    before do
+      result.stub(success?: false, stderr: '')
+    end
+
+    it { should warn }
+  end
+
+  context 'when git lfs hook exits successfully' do
+    before do
+      result.stub(success?: true, stderr: '')
+    end
+
+    it { should pass }
+  end
+
+  context 'when git lfs hook exits unsuccessfully' do
+    before do
+      # First for checking that git-lfs is on path, second for calling the hook itself
+      result.stub(:success?).and_return(true, false)
+      result.stub(:stderr).and_return('', 'error: failed to push some refs')
+    end
+
+    it { should fail_hook }
+  end
+end