Rollup merge of #121781 - RalfJung:bootstrap-fmt, r=clubby789

bootstrap/format: send larger batches to rustfmt

This helps on systems with low core counts. To benchmark this I made a lot of files be modified:
```
for FILE in $(find compiler/ -name "*.rs"); do echo "// end of the file" >>$FILE; done
```
Then I ran
```
hyperfine "./x.py fmt -j1" -w 1 -r 4
```
Before this patch:
```
Benchmark 1: ./x.py fmt -j1
  Time (mean ± σ):      3.426 s ±  0.032 s    [User: 4.681 s, System: 1.376 s]
  Range (min … max):    3.389 s …  3.462 s    4 runs
```
With this patch:
```
Benchmark 1: ./x.py fmt -j1
  Time (mean ± σ):      2.530 s ±  0.054 s    [User: 4.042 s, System: 0.467 s]
  Range (min … max):    2.452 s …  2.576 s    4 runs
```

0 files changed, 0 insertions, 0 deletions