[FE] Code Analytics- Files committed together

This issue adds a graph chart to the Code Analytics page as described in https://gitlab.com/gitlab-org/gitlab-ee/issues/12648

Requirements

• Implement a Graph Chart in gitlab-ui, see gitlab-ui#375 (closed)
• Integrate the chart into the Code Analytics page and its architecture (Vuex store)
• Populate the chart with data from the API

Questions

• How does the state on the FE look like?
  • I suppose we will save the currently active filters on Vuex store similar to Productivity Analytics state?
• What's the API endoint that can be used for querying data?
• Which query params are we going to use for the API?
  • project_id (see https://gitlab.com/gitlab-org/gitlab-ee/issues/12648#note_197192043)
  • date range (see https://gitlab.com/gitlab-org/gitlab-ee/issues/12648#note_197192043)
  • anything else?
• How do we deal with long file names in the chart?