When an existing install at $INSTALL_DIR has an unmerged index (files in a
"needs merge" state left by a previously interrupted update), the update path
ran `git stash` then `git checkout <branch>`. On a conflicted index `git stash`
aborts with "could not write index" and `git checkout` then aborts with "you
need to resolve your current index first" — surfacing to desktop/bootstrap
users as `git checkout main failed (exit 1)` and failing the whole install at
the repository stage.
Mirror the `hermes update` Python path (#4735): detect unmerged entries with
`git ls-files --unmerged` and clear the conflict state with `git reset` before
stashing. Working-tree changes are still captured by the subsequent stash, so
nothing is discarded; only the index-level conflict markers are dropped, which
lets the checkout proceed.
Fixed in both installers (install.sh and install.ps1) so the Windows GUI
installer and the POSIX one share the same recovery behavior.
573b964dc7