This PR changes how we version going forward in the following ways:

* mark-new-version.sh is changed to a new policy of just splitting
branches, rather than the old backmerge policy, as discussed in
vX.Y.0, and a tag for vX.(Y+1).0-alpha.0 back to master.

* I eliminated PRs back to master by making the version/base.go
gitVersion and gitCommit just be `export-subst`. I testing that this
works with GitHub's source export tarballs. There's no reason to
bother with forcing the version into `base.go` (especially twice). The
tarball versions outside a git tree aren't perfect (master looks like
"v0.0.0+hash", and the release branches look more accurate), but our
build contract has never allowed that version is perfect in this
situation, so I think we can relax this.

* That master tag gets picked up by "git describe" on master, so e.g.
master would have immediately become v1.1.0-alpha.0

* In order to be more semVer compatible, the gitVersion field for the
master branch now looks something like 1.1.0-alpha.0.6+84c76d11.
This is a tiny translation of the "git describe". I did this because
there are a ton of consumers out there of the "gitVersion" field
expecting it to be the full version, but it would be nice if this
field were actually semver compliant. (1.1.0-alpha.0-6-84c76d11
is, but it's not *usefully* so.)

Fixes #11495