newvers: tweak uname to be more useful

The current uname is branch-cXXXX-gHASH

Three changes to make uname more useful.

1. Move from using git rev-list --count to git rev-lis --count --first-parent since that gives a better, incrementing number.
2. Report this count as 'nXXXXX' rather than 'cXXXXX' because c is part of a hash and we've changed the sematnics of XXXXX
3. Remove g to make HASH cut and pastable.

Durting review, #1 & #3 had the largest consensus. There was a diversity of
opinion on #2, but on the whole it was positive so I'll acknowledge the dissent,
but move forward with something seems to have support since the dissent was all
about what letter to use where I chose 'n'.

MFC After: 3 days
Reviewed by: rgrimes, emaste (earlier version)
Differential Revision: https://reviews.freebsd.org/D28338

(cherry picked from commit 8a51f14a7833fd14e1f125e63a0af9d260dcd287)