qf: fix shellcheck warning

In qf line 411:
git reset --hard $(git rev-parse --abbrev-ref @{u})
 ^-- SC1083: This { is literal. Check expression (missing ;/\n?) or quote it.
 ^-- SC1083: This } is literal. Check expression (missing ;/\n?) or quote it.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Fixes: 0db98111 ("qf: Make qf_pull -f forcibly clean")
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>