We are never supposed to merge anything. We are supposed to just follow
upstream. pull merges, so it's just the wrong tool.
Instead, fetch and reset.
It is not a problem to always reset, as user already cannot rely on
repository's survival across updates: if there is any error reported by
git command, the repository will be wiped out. So resetting will not do
any more harm than what can already happen.
Also, properly quote paths to protect against spaces (in both command and
update-command).