Committers Guide: update the ports-specific sections to git.

• Remove notes that the migration is in progress
• Remove old branch names and URLs
• Remove mention of psvn
• Remove section about re-adding a deleted port, git add will do the right thingn natively (most of the times).
• Remove section about repo-copies, git has no copy command.
• Simplify instructions for moving a port.
• Remove section about the mfh tool. Merges are now just cherry-picks.

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