Update gitlab-grit dependency to 2.7.1

Changes to gitlab-grit since 2.6.5 included below.

== 2.7.1
  * Suppress 'unkown header' warnings
  * Add process.out to CommandFailed error

== 2.7.0
  * Remove the `chdir:` option for thread-safety
  * Automatically set the `--work-tree=` Git option

== 2.6.9
  * Fix commit diff issue. It shows empty diff for commit when files
  * have changed mode

== 2.6.8
  * Fix problem with generating archives

== 2.6.7
  * Rescue compatibility errors

== 2.6.6
  * Improve encoding
Merge pull request #1 from jacobvosmaer/update_gitlab-grit

Update gitlab-grit dependency to 2.7.1