Add devel/gitinspector.

gitinspector is a statistical analysis tool for git repositories. The
defaut analysis shows general statistics per author, which can be
complemented with a timeline analysis that shows the workload and
activity of each author. Under normal operation, it filters the
results to only show statistics about a number of given extensions and
by default only includes source files in the statistical analysis.

Differential Revision: https://reviews.freebsd.org/D1529
Reviewed by: bapt
Approved by: bapt