Add the functionality for commit search.

Ensure that the preview accept header is set properly

lib/octokit/client/search.rb

@@ -20,6 +20,21 @@ def search_code(query, options = {})
         search "search/code", query, options
       end
 
+      # Search commits
+      #
+      # @param query [String] Search terms and qualifiers
+      # @param options [Hash] Sort and pagination options
+      # @option options [String] :sort Sort field
+      # @option options [String] :order Sort order (asc or desc)
+      # @option options [Integer] :page Page of paginated results
+      # @option options [Integer] :per_page Number of items per page
+      # @return [Sawyer::Resource] Search results object
+      # @see https://developer.github.com/v3/search/#search-commits
+      def search_commits(query, options = {})
+        options = ensure_api_media_type(:commit_search, options)
+        search "search/commits", query, options
+      end
+
       # Search issues
       #
       # @param query [String] Search term and qualifiers