R1:aa310230b6bc

Detect moves and copies with some unchanged lines as moves or copies

Summary:
Ref T1266. We won't detect a move/copy if fewer than 3 lines are changed.

However, you may move a block like:

Complicated Line A
Trivial Line B
Complicated Line C

...where "Trivial Line B" is something like a curly brace. If you move this block somewhere that happened to previously have a similar trivial curly brace line, we won't be able to find 3 contiguous added lines in order to detect the copy/move.

Instead…