devel/freebsd-git-devtools: Simple Git tools from src/tools/tools/git

Rename from devel/git-arc to indicate that more tools from
src/tools/tools/git/ may be included. The only additional tool for now
is mfc-candidates, which, as the name suggests, is a tool to find MFC
candidates. Under a ports tree, mfc-candidates finds commits to merge
to quarterly.

Approved by: emaste, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35165