devel/git-lfs: update to v2.6.0

• adopted the port;
• made better use of port's Go framework;
• removed 'shells/bash' and 'devel/gmake' from BUILD_DEPENDS;
• added 'devel/git' to RUN_DEPENDS;
• added 'pkg-message' with info from upstream on how to setup LFS;
• added new 'MANPAGES' option.

Approved by: araujo (mentor), mat
Differential Revision: https://reviews.freebsd.org/D17904