Switch upstream git repository.

Previously portable OpenSSH has synced against a conversion of OpenBSD's
CVS repository made using the git cvsimport tool, but this has become
increasingly unreliable.

As of this commit, portable OpenSSH now tracks a conversion of the
OpenBSD CVS upstream made using the excellent cvs2gitdump tool from
YASUOKA Masahiko: https://github.com/yasuoka/cvs2gitdump

cvs2gitdump is considerably more reliable than gitcvsimport and the old
version of cvsps that it uses under the hood, and is the same tool used
to export the entire OpenBSD repository to git (so we know it can cope
with future growth).

These new conversions are mirrored at github, so interested parties can
match portable OpenSSH commits to their upstream counterparts.

https://github.com/djmdjm/openbsd-openssh-src
https://github.com/djmdjm/openbsd-openssh-regress

An unfortunate side effect of switching upstreams is that we must have
a flag day, across which the upstream commit IDs will be inconsistent.
The old commit IDs are recorded with the tags "Upstream-ID" for main
directory commits and "Upstream-Regress-ID" for regress commits.

To make it clear that the commit IDs do not refer to the same
things, the new repository will instead use "OpenBSD-ID" and
"OpenBSD-Regress-ID" tags instead.

Apart from being a longwinded explanation of what is going on, this
commit message also serves to synchronise our tools with the state of
the tree, which happens to be:

OpenBSD-ID: 9c43a9968c7929613284ea18e9fb92e4e2a8e4c1
OpenBSD-Regress-ID: b33b385719420bf3bc57d664feda6f699c147fef

.skipped-commit-ids

@@ -1,3 +1,7 @@
+# Commit IDs against the new CVS->GIT translation go here (and delete this line)
+
+Old upstream tree:
+
 321065a95a7ccebdd5fd08482a1e19afbf524e35	Update DH groups
 d4f699a421504df35254cf1c6f1a7c304fb907ca	Remove 1k bit groups
 aafe246655b53b52bc32c8a24002bc262f4230f7	Remove intermediate moduli