[REVIEW] 16719 nightly should be able to create and update build trees from git

[REVIEW] 16719 nightly should be able to create and update build trees from git

[Bill Sommerfeld]

Issue: https://www.illumos.org/issues/16719
CR: https://code.illumos.org/c/illumos-gate/+/3642
Diff: 
https://code.illumos.org/~diff/bdc184fadd01f4238ca20d4ec117160a1383574e

The nightly script has long had a facility to automatically populate
an initially empty build tree, and then incrementally pull changes
into it.

This worked with both teamware and mercurial, but when we switched
from mercurial to git, no changes were made for git, because it was
unclear what the right way to handle merge conflicts during the pull.

For a while I've had private changes to nightly that fix this, in an
admittedly opinionated way: I don't let it attempt to merge.  Instead,
it uses "git switch" to set the build branch to the tip of the source
branch.

I'd like to integrate these changes into illumos-gate so that others
can use them.

No change to default nightly behavior is expected as the default
illumos environment file includes the 'n' option.
I've updated the man page and the default environment file to match.
In particular I've added a "WARNINGS" section explaining how this can
potentially lead to changes getting lost or hidden if you expect it to
behave like the way "bringover" used to behave.

Thanks in advance for the review.

					- Bill