An interrupted previous clone leaves the install dir's .git present but with
no initial commit. rev-parse --is-inside-work-tree and git status both still
succeed there, so the installer entered the update path and ran `git stash`,
which aborts with "You do not have the initial commit yet" and failed the
desktop install at the "Cloning Hermes repository" stage.
- install.ps1: add a `git rev-parse --verify HEAD` probe to the repo-validity
check so a commit-less checkout is treated as broken and re-cloned fresh.
- install.sh: mirror it at the top of clone_repo() — drop a partial checkout
with no resolvable HEAD so the fresh-clone path handles it (POSIX parity).
Fixes#40998
fc0900d120