R1:9cf3b3bbf806

Count lines in build log slices more cheaply

Summary:
See PHI766. Ref T13164. Build log chunk processing does a `preg_split()` on slices, but this isn't terribly efficient.

We can get the same count more cheaply by just using `substr_count()` a few times.

(I also tried `preg_match_all()`, which was between the two in speed.)

Test Plan:
- Used `bin/harbormaster rebuild-log --id X --force` to rebuild logs. Verified that the linemap is identical before/after this change.
- Saw local time for the 18MB log…