opam pin add doesn't upgrade local git repository when the package was already pinned on the same location

```
$ opam --version
2.5.0
$ cd ./some-local-project
$ opam pin
$ opam pin .
[... install]
$ git pull origin branch
[... changes get pulled]
$ opam pin .
Already up-to-date.
Nothing to do.
$ opam upgrade
Nothing to do.
$ opam reinstall <pkg>
[pkg] synchronised (no changes)
[...]
$ # the new changes are now installed
```