build: Support gotHEADmain

Add code for using got(1), matching how git(1) is currently used:
1. Determining the git branch,
2. Determining the checked-out hash,
3. Determining the commit count,
4. Determining the timestamp of the most recent commit,
5. Determining if the tree is "dirty",
6. Generating lists of files for the src and src-sys packages, and
7. Excluding .got directories from src.txz and ports.txz.

The release engineer hopes to use this functionality in release builds,
since got's minimalism makes it a far more manageable build tool than
git.

Reviewed by:	naddy
MFC after:	1 week
Sponsored by:	Amazon
Differential Revision:	https://reviews.freebsd.org/D60606

0 files changed, 0 insertions, 0 deletions