Add helper method HookContext::PreCommit#amend?

Returns true if the current hook run was initiated by amending
a commit, false otherwise.

Author: jawshooah

lib/overcommit/hook_context/pre_commit.rb

@@ -7,7 +7,21 @@ module Overcommit::HookContext
   # This includes staged files, which lines of those files have been modified,
   # etc. It is also responsible for saving/restoring the state of the repo so
   # hooks only inspect staged changes.
-  class PreCommit < Base
+  class PreCommit < Base # rubocop:disable ClassLength
+    # Returns whether this hook run was triggered by `git commit --amend`
+    def amend?
+      if @amended.nil?
+        cmd = Overcommit::Utils.parent_command
+        @amended = !(/--amend/ =~ cmd).nil?
+
+        amend_alias = `git config --get-regexp '^alias\\.' '--amend'`.
+          slice(/(?<=alias\.)\w+/)
+
+        @amended ||= !(/git #{amend_alias}/ =~ cmd).nil? unless amend_alias.nil?
+      end
+      @amended
+    end
+
     # Stash unstaged contents of files so hooks don't see changes that aren't
     # about to be committed.
     def setup_environment

spec/overcommit/hook_context/pre_commit_spec.rb

@@ -7,6 +7,41 @@
   let(:input) { double('input') }
   let(:context) { described_class.new(config, args, input) }
 
+  describe '#amend?' do
+    subject { context.amend? }
+
+    context 'when amending a commit using `git commit --amend`' do
+      before do
+        Overcommit::Utils.stub(:parent_command).and_return('git commit --amend')
+      end
+
+      it { should == true }
+    end
+
+    context 'when amending a commit using a git alias' do
+      around do |example|
+        repo do
+          `git config alias.amend 'commit --amend'`
+          example.run
+        end
+      end
+
+      before do
+        Overcommit::Utils.stub(:parent_command).and_return('git amend')
+      end
+
+      it { should == true }
+    end
+
+    context 'when not amending a commit' do
+      before do
+        Overcommit::Utils.stub(:parent_command).and_return('git commit')
+      end
+
+      it { should == false }
+    end
+  end
+
   describe '#setup_environment' do
     subject { context.setup_environment }