Test code:
```py
import pyperf

from mpmath import pi
from mpmath.libmp.libmpf import format_hexadecimal as fd

runner = pyperf.Runner()
pi = pi._mpf_
runner.bench_func('fd(pi, -1)', fd, pi)
runner.bench_func('fd(pi, +5)', fd, pi, 5)
```

Results:
| Benchmark      | master  | patch                 |
|----------------|:-------:|:---------------------:|
| fd(pi, -1)     | 38.9 us | 17.1 us: 2.27x faster |
| fd(pi, +5)     | 39.7 us | 18.7 us: 2.13x faster |
| Geometric mean | (ref)   | 2.20x faster          |