R1:7affae934594

Detect copied code by own algorithm

Summary:
Required for D2321.
Deprecates D2320.
Uses algorithm described at D2320#16.

Complexity of this algorithm would be `O(N)` (`N` stands for number of lines) in most cases.
The worst case is `O(A*F)` (`A` stands for number of added lines, `F` for number of colliding lines) but it should be pretty rare. Real-world example is 100 modified files with moved license block (15 lines) in each. This will require 1500*100 comparisons because the algorithm will be trying to…