gstack-pregate --base <sha> refused with "fix: git fetch origin <sha>
--depth=1". That fetch can never create origin/<sha>: a short SHA fails
with "couldn't find remote ref", and a full SHA only reaches FETCH_HEAD.
A fork clone hits the same wall with --base upstream/main.
When origin/<base> does not resolve but <base> names a commit here and
no local branch (a SHA, a tag, another remote's ref), the refusal now
says --base takes a branch name on origin and suggests origin/HEAD's
branch. A local branch, or a name that resolves nowhere, keeps the
fetch fix: that is the unfetched or shallow checkout it repairs. Same
code, same exit 3, and pin files are still read only from the reviewed
origin/<base>.
Fixes#3269
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
