Backport #22611
The code for checking if a commit has caused a change in a PR is
extremely inefficient and affects the head repository instead of using a
temporary repository.
This PR therefore makes several significant improvements:
* A temporary repo like that used in merging.
* The diff code is then significant improved to use a three-way diff
instead of comparing diffs (possibly binary) line-by-line - in memory...
Ref #22578
Signed-off-by: Andrew Thornton <art27@cantab.net>
a9ba7379fe