devel/git: Split into subpackages

Removed CVS, GUI, PERFORCE and SUBVERSION options and also gui and svn
FLAVORS, and create 4 new subports:

devel/git-cvs
devel/git-gui
devel/git-p4
devel/git-svn

All these packages depend of devel/git and install only additional files
and manpages. This work is based on initial patch submitted by grembo@i
at review D30238.

PR: 251090
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32369