linelog: fix infinite loop vulnerability Checking `len(lines)` is not a great way of detecting infinite loops, as demonstrated in the added test. Therefore check instruction count instead. The original C implementation does not have this problem. There are a few other places where the C implementation enforces more strictly, like `a1 <= a2`, `b1 <= b2`, `rev > 0`. But they are optional. Test Plan: Add a test. The old code forces the test to time out. Differential Revision: https://phab.mercurial-scm.org/D4151