The current TestPatch conflict code uses a plain git apply which does not properly
account for 3-way merging. However, we can improve things using `git read-tree -m` to
do a three-way merge then follow the algorithm used in merge-one-file. We can also use
`--patience` and/or `--histogram` to generate a nicer diff for applying patches too.
Fix #13679
Fix #6417
Signed-off-by: Andrew Thornton <art27@cantab.net>
