fix: use git pull --ff-only in update/install to avoid divergent branch error

Fresh installs without pull.rebase configured hit a git error when
running hermes update because git doesn't know how to reconcile
divergent branches. --ff-only is the right strategy: it works for the
normal case (local branch is behind remote) and fails cleanly if the
user somehow has local commits, rather than silently rebasing them.

scripts/install.sh

@@ -577,7 +577,7 @@ clone_repo() {
 
             git fetch origin
             git checkout "$BRANCH"
-            git pull origin "$BRANCH"
+            git pull --ff-only origin "$BRANCH"
 
             if [ -n "$autostash_ref" ]; then
                 local restore_now="yes"