Handbook: adjust for the various git migrations.

• prefer to install git as a package over building it oneself.
• update dates in in the introduction
• let some Subversion URLs move to the src repository, as it is the only repository with an Subversion exporter active.
• let the user install git instead of subversion for doing ports work.

Reviewed by: emaste, mat
Differential Revision: https://reviews.freebsd.org/D29450