--null-data seems to inhibit some optimizations

[BurntSushi]

To reproduce, first create a large binary file:

dd status=progress if=/dev/zero count=10240000 bs=1024 > ./binary-test.bin ; echo 'ZQZQZQZQZQ' >> ./binary-test.bin ; dd if=/dev/zero count=10240000 bs=1024 status=progress >> ./binary-test.bin

And then try to search it with --null-data:

$ time rg --null-data --no-mmap ZQZQZQZQZQ binary-test.bin > out.rg

real    2:00.24
user    1:57.95
sys     2.246
maxmem  19 MB
faults  0

It should not take that long. A quick peak at perf top while the above process was running reveals it is doing a line-by-line search, probably because of some faulty reasoning about the line terminator in this case. Since most of the file is just line terminators, this spends a lot of time iterating over lines.

I found this as part of #2831.