Fix "Average 0 commits per day" by preventing integer division.

Open username-removed-716864 requested to merge mamrehn1/gitlab-ce:patch-5 into master

With a commit count smaller than the number of days a repository is in existence the current version shows "Average 0 commits per day" on <user_name>/<repository_name>/graphs/master/commits.

This should be changed back to i.e "Average 0.35 commits per day" to be meaningful and even correct.