release: sync 'git count' logic with newvers.sh

Sync determining the git count with newvers.sh by adding the
--first-parent argument. This ensures uname(1) reflects the
file name for snapshots.

Reported by: Mark Millard
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit e5c6913a8d23ebe49c9e8f95c5236c7a0d8dbac5)