The installer's re-run path fetched `origin <branch>` by name, then checked
the branch out and fast-forwarded to origin/<branch>. On a checkout an older
installer made with `--depth 1 --single-branch --branch <tag>`, the remote's
only refspec maps the tag: the fetch wrote FETCH_HEAD but no origin/<branch>,
and `git checkout main` failed with "pathspec 'main' did not match".
Fetch by explicit refspec (as `hermes update` now does), and when there is no
local branch yet create it at the fetched tip — checkout's own branch guess
ignores remote refs the configured refspec does not map. The remote's fetch
config is left as the user has it. Same change in install.sh and install.ps1.
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