Add merge_base support (#405)

* Add support for merge_base API endpoint

* Update merge_base stub to use the correct stubbed response from docs

* Update comments

lib/gitlab/client/repositories.rb

@@ -57,5 +57,18 @@ def compare(project, from, to)
       get("/projects/#{url_encode project}/repository/compare", query: { from: from, to: to })
     end
     alias repo_compare compare
+
+    # Get the common ancestor for 2 refs (commit SHAs, branch names or tags).
+    #
+    # @example
+    #   Gitlab.merge_base(42, ['master', 'feature/branch'])
+    #   Gitlab.merge_base(42, ['master', 'feature/branch'])
+    #
+    # @param [Integer, String] project The ID or URL-encoded path of the project.
+    # @param [Array] refs Array containing 2 commit SHAs, branch names, or tags.
+    # @return [Gitlab::ObjectifiedHash]
+    def merge_base(project, refs)
+      get("/projects/#{url_encode project}/repository/merge_base", query: { refs: refs })
+    end
   end
 end

spec/fixtures/merge_base.json

@@ -0,0 +1,14 @@
+{
+    "id": "1a0b36b3cdad1d2ee32457c102a8c0b7056fa863",
+    "short_id": "1a0b36b3",
+    "title": "Initial commit",
+    "created_at": "2014-02-27T08:03:18.000Z",
+    "parent_ids": [],
+    "message": "Initial commit\n",
+    "author_name": "Dmitriy Zaporozhets",
+    "author_email": "[email protected]",
+    "authored_date": "2014-02-27T08:03:18.000Z",
+    "committer_name": "Dmitriy Zaporozhets",
+    "committer_email": "[email protected]",
+    "committed_date": "2014-02-27T08:03:18.000Z"
+}

spec/gitlab/client/repositories_spec.rb

@@ -93,4 +93,22 @@
       expect(@diff.diffs.last['new_path']).to eq 'files/js/application.js'
     end
   end
+
+  describe '.merge_base' do
+    before do
+      stub_get('/projects/3/repository/merge_base', 'merge_base')
+        .with(query: { refs: %w[master feature] })
+      @response = Gitlab.merge_base(3, %w[master feature])
+    end
+
+    it 'gets the correct resource' do
+      expect(a_get('/projects/3/repository/merge_base')
+        .with(query: { refs: %w[master feature] })).to have_been_made
+    end
+
+    it 'gets common ancestor of the two refs' do
+      expect(@response).to be_kind_of Gitlab::ObjectifiedHash
+      expect(@response.id).to eq '1a0b36b3cdad1d2ee32457c102a8c0b7056fa863'
+    end
+  end
 end