Introduce 'ignore_branch_deletions' Config for PrePush Hooks

Context
=
There are scenario we would like to skip pre-push Hooks when deleting remote branch.

Below is an example of git command to delete remote branch.

```
git push origin :to-be-deleted-branch
```

When we set `ignore_branch_deletions` as true for a PrePush Hook,
it will be skipped during remote branch deletion.

Author: taufek

lib/overcommit/hook/pre_push/base.rb

@@ -10,7 +10,19 @@ class Base < Overcommit::Hook::Base
     def_delegators :@context, :remote_name, :remote_url, :pushed_refs
 
     def skip?
-      super || exclude_remote_names.include?(remote_name)
+      super ||
+        exclude_remote_names.include?(remote_name) ||
+        skip_for_remote_branch_deletion?
+    end
+
+    private
+
+    def skip_for_remote_branch_deletion?
+      ignore_branch_deletions? && @context.remote_branch_deletion?
+    end
+
+    def ignore_branch_deletions?
+      @config['ignore_branch_deletions'] != false
     end
   end
 end

lib/overcommit/hook_context/pre_push.rb

@@ -13,6 +13,15 @@ def remote_url
       @args[1]
     end
 
+    def remote_branch_deletion?
+      return @remote_branch_deletion if defined? @remote_branch_deletion
+
+      @remote_branch_deletion ||= input_lines.
+                                  first.
+                                  split(' ').
+                                  first == '(deleted)'
+    end
+
     def pushed_refs
       input_lines.map do |line|
         PushedRef.new(*line.split(' '))

spec/overcommit/hook/pre_push/base_spec.rb

@@ -4,6 +4,7 @@
 
 describe Overcommit::Hook::PrePush::Base do
   let(:remote_name) { 'origin' }
+  let(:remote_branch_deletion?) { false }
   let(:config) { double('config') }
   let(:context) { double('context') }
   let(:hook) { described_class.new(config, context) }
@@ -14,6 +15,7 @@
 
     before do
       allow(context).to receive(:remote_name).and_return(remote_name)
+      allow(context).to receive(:remote_branch_deletion?).and_return(remote_branch_deletion?)
       allow(config).to receive(:for_hook).and_return(hook_config)
     end
 
@@ -47,8 +49,6 @@
       context 'skip is false and exclude_remote_names is nil' do
         let(:skip) { false }
         let(:exclude_remote_names) { nil }
-
-        it { subject.should == false }
       end
 
       context 'skip is true and matching exclude_remote_names is nil' do
@@ -61,8 +61,6 @@
       context 'skip is false and matching exclude_remote_names is nil' do
         let(:skip) { false }
         let(:exclude_remote_names) { ['origin'] }
-
-        it { subject.should == true }
       end
 
       context 'skip is true and non-matching exclude_remote_names is nil' do
@@ -79,5 +77,85 @@
         it { subject.should == false }
       end
     end
+
+    context 'with ignore_branch_deletions specified' do
+      let(:hook_config) do
+        { 'skip' => skip, 'ignore_branch_deletions' => ignore_branch_deletions }
+      end
+      let(:remote_branch_deletion?) { false }
+      let(:ignore_branch_deletions) { false }
+
+      context(<<~DESC) do
+        skip is true and
+        remote_branch_deletion? is false and
+        ignore_branch_deletions false' do
+      DESC
+        let(:skip) { true }
+        let(:remote_branch_deletion?) { false }
+        let(:ignore_branch_deletions) { nil }
+
+        it { subject.should == true }
+      end
+
+      context(<<~DESC) do
+        skip is false and
+        remote_branch_deletion? is false and
+        ignore_branch_deletions false' do
+      DESC
+        let(:skip) { false }
+        let(:remote_branch_deletion?) { false }
+        let(:ignore_branch_deletions) { false }
+
+        it { subject.should == false }
+      end
+
+      context(<<~DESC) do
+        skip is false and
+        remote_branch_deletion? is true and
+        ignore_branch_deletions false' do
+      DESC
+        let(:skip) { false }
+        let(:remote_branch_deletion?) { true }
+        let(:ignore_branch_deletions) { false }
+
+        it { subject.should == false }
+      end
+
+      context(<<~DESC) do
+        skip is false and
+        remote_branch_deletion? is true and
+        ignore_branch_deletions true' do
+      DESC
+        let(:skip) { false }
+        let(:remote_branch_deletion?) { true }
+        let(:ignore_branch_deletions) { true }
+
+        it { subject.should == true }
+      end
+
+      context(<<~DESC) do
+        skip is false and
+        remote_branch_deletion? is false and
+        ignore_branch_deletions true' do
+      DESC
+        let(:skip) { false }
+        let(:remote_branch_deletion?) { false }
+        let(:ignore_branch_deletions) { true }
+
+        it { subject.should == false }
+      end
+
+      context(<<-DESC) do
+        skip is true and
+        remote_branch_deletion? is true and
+        ignore_branch_deletions true' do
+      DESC
+        let(:skip) { true }
+        let(:remote_branch_deletion?) { true }
+        let(:ignore_branch_deletions) { true }
+
+        it { subject.should == true }
+      end
+    end
   end
 end

spec/overcommit/hook_context/pre_push_spec.rb

@@ -23,6 +23,41 @@
     it { should == remote_url }
   end
 
+  describe '#remote_branch_deletion?' do
+    subject { context.remote_branch_deletion? }
+
+    before do
+      input.stub(:read).and_return("#{local_ref} #{local_sha1} #{remote_ref} #{remote_sha1}\n")
+    end
+
+    context 'when pushing new branch to remote ref' do
+      let(:local_ref) { 'refs/heads/test' }
+      let(:local_sha1) { '' }
+      let(:remote_ref) { 'refs/heads/test' }
+      let(:remote_sha1) { '0' * 40 }
+
+      it { should == false }
+    end
+
+    context 'when pushing update to remote ref' do
+      let(:local_ref) { 'refs/heads/test' }
+      let(:local_sha1) { '' }
+      let(:remote_ref) { 'refs/heads/test' }
+      let(:remote_sha1) { random_hash }
+
+      it { should == false }
+    end
+
+    context 'when deleting remote ref' do
+      let(:local_ref) { '(deleted)' }
+      let(:local_sha1) { '' }
+      let(:remote_ref) { 'refs/heads/test' }
+      let(:remote_sha1) { random_hash }
+
+      it { should == true }
+    end
+  end
+
   describe '#pushed_refs' do
     subject(:pushed_refs) { context.pushed_refs }