New port: devel/git-subrepo: Git command for managing subrepositories

This git command "clones" an external git repo into a subdirectory of
your repo. Later on, upstream changes can be pulled in, and local
changes can be pushed back.

WWW: https://github.com/ingydotnet/git-subrepo

Reviewed by: brooks
Approved by: bdrewery
Feedback from: mat
Differential Revision: https://reviews.freebsd.org/D20189