Change in the uname

[Warner Losh]

Now that we have some experience with git, I'd like to propose we tweak the
uname a little.

Right now it is branch-cXXXXX-gHASH.

First problem: cXXXX isn't linear. It suffers the same issues that git log
does in that it includes all changes from merge commits back to a common
ancestor. However, branch~1234 (1234 commits before the tip of branch)
doesn't count those.

Solution: change this to vXXXX and add --first-parent to the git rev-list
command. This will give us something we can use with the branch~number
notation to track down versions more easily and allow people moving forward
100 commits to see the v number increment by 100 commits. I'm changing c->v
here for two reasons. (1) c is a valid hex character so cXXXXX looks like a
hash and (2) we're changing the semantic meaning of XXXXX, so this will
make things unambiguous. This will also make it easier to find the revision
with the branch~123 notation should that ever become necessary (and the v
number would decrement by 123 when people do this).

Granted, this isn't a super-huge deal since we have the hash, but it will
make things a bit more predictable and eliminate at least one source of
confusion.

Second problem: gHASH isn't cut-and-pastable. And it makes it look like a
git describe token when it isn't.

Solution: delete the 'g'.

So the new uname will be branch-vXXXXX-HASH.

I'd like to push this to stable/13 as well before the release so we're not
stuck with two versions.

See https://reviews.freebsd.org/D28338 for the proposed change.

Comments?

Warner