added Gitlab::Git::Compare for git compare feature

lib/gitlab/git/compare.rb

+module Gitlab
+  module Git
+    class Compare
+      attr_accessor :commits, :commit, :diffs, :same
+
+      def initialize(repository, from, to)
+        @commits, @diffs = [], []
+        @commit = nil
+        @same = false
+
+        return unless from && to
+
+        first = repository.commit(to.try(:strip))
+        last = repository.commit(from.try(:strip))
+
+        return unless first && last
+
+        if first.id == last.id
+          @same = true
+          return
+        end
+
+        @commit = Commit.new(first)
+
+        @commits = repository.commits_between(last.id, first.id)
+        @commits = @commits.map { |c| Commit.new(c) }
+
+        @diffs = if @commits.size > 100
+                   []
+                 else
+                   repository.repo.diff(last.id, first.id) rescue []
+                 end
+      end
+    end
+  end
+end
+