Partially revert r346018 and use the if/then construct instead of shell.

There are a couple of places in the tree that directly parse the newvers.sh
script looking for the BRANCH variable. I found two locations, one in
release/Makefile and the other in bin/freebsd-version/Makefile.

While there is a good argument that BRANCH_OVERRIDE should properly
propagate in those circumstances and the new behavior is thus better, the
reality is this change broke freebsd-update's ability to find timestamps in
binaries and resulted in a large number of gratuitous changes.

Reported by: freebsd-update
Discussed with: cperciva
MFC after: 1 day