opam update completely reloads the repository

[dra27]

This is most noticeable on Windows, where the file system I/O between opam / OCaml is effectively slower, but the problem can be seen in less-cached environments - e.g. in a Docker container on Linux.

For a git remote, opam is able to short-circuit reloading the entire repository when it detects that there are no changes (this is a good reason for using git remotes, as it's a faster check than for the https remote). However, if there are any changes, the entire repository is reloaded.

It seems we could be fairly easily doing something a little better, and possibly also for the https remote on update. The update is actually applied by means of a patch (even for git remotes), which means that opam always knows exactly which files have changed.

Related to @kit-ty-kate's work on #5741 (and also #5648).

[kit-ty-kate]

Could be done using the diff implementation in #5892

[kit-ty-kate]

I had a look at this again and it would require #5892 (and also use this diff implementations for VCSs) or someone needs to go on a deep dive and find a way to fix hannesm/patch#17 that matches every implementations of diff