For a long time Gitea has tested PR patches using a git apply --check
method and in fact prior to the introduction of a read-tree assisted
three-way merge in go-gitea#18004, this was the only way of checking patches.

Since go-gitea#18004, the git apply --check method has been a fallback method,
only used when the read-tree method has detected a conflict. The
read-tree assisted three-way merge method is much faster and less
resource intensive method of detecting conflicts. go-gitea#18004 kept the git
apply method around because it was thought possible that this fallback
might be able to rectify conflicts that the read-tree three-way merge
detected. I am not certain if this could ever be the case.

Given the uncertainty here and the now relative stability of the
read-tree method - this PR makes using this fallback optional and
disables it by default. The hope is that users will not mention any
significant difference in conflict detection and we will be able to
remove the git apply fallback in future, and/or improve the read-tree
three-way merge method to catch any conflicts that git apply method
might have been incorrectly detecting.

(See https://github.com/go-gitea/gitea/issues/22083\#issuecomment-1347961737)

Ref go-gitea#22083

Signed-off-by: Andrew Thornton <[email protected]>