Fix Issue 707 (in gitlab): Indent unfolded code 1 character

Created by: Leibniz137

This is a fix for https://gitlab.com/gitlab-org/gitlab-ce/issues/707

Note that if passed an invalid value is passed as the 'input' parameter (e.g. -1), Gitlab will 500. Not sure if this is acceptable behavior.

Manually tested GET'ing diffs with a valid value passed as the 'indent' parameter. Also tested GET'ing a diff without specifying the 'indent' param, confirmed that no indentation was added.

Before unfold:

After unfold: