R1:d44a5fa93374

In Git, only use "--find-copies-harder" on small diffs

Summary:
Ref T10423. This flag can cause `git diff` to take an enormously long time (the problem case was a 5M line, 20K file commit).

Instead:

- Run without the flag first.
- If that shows that the diff is definitely small, try again with the flag.
- If that works, return the slower, better output.
- If the fast diff affects too many paths or generating the slow diff takes too long, return the faster, slightly worse output.

The quality of…