Fix github action "performance-8.0" when PR is from a fork.

[pserwylo]

I noticed in another PR of mine that the performance-8.0 test was failing, where it was passing for many other PRs. On inspection, there were others PRs where this failed, and all of them were where the PR came from a fork rather than a branch in this repo.

This is because there is an assumption in the workflow that the PR is from the same origin as the base it is being merged into. This is fixed so that instead of doing a git fetch origin, it now fetches from the clone_url of the PR head.

Subsequent to fetching commits from that repository, it then clones based on sha1 instead of branch name. This is because we didn't name the remote when performing a fetch, so can't use the syntax remote-name/ref (e.g. "myusername/fix-issue-12").

We could equally go and add a remote before fetching, then fetch from that remote (e.g git add remote head ... && git fetch head) which would then let us git merge head/${{ ref }} - it doesn't really matter which way we do it.

[szaimen]

Thanks

[skjnldsv]

From https://github.com/pserwylo/nextcloud-server
 * branch                  HEAD       -> FETCH_HEAD
warning: 9d07ec5a96d7995785f6d3d377024ba09977cb91:.gitmodules, multiple configurations found for 'submodule.3rdparty.path'. Skipping second one!
warning: 9d07ec5a96d7995785f6d3d377024ba09977cb91:.gitmodules, multiple configurations found for 'submodule.3rdparty.url'. Skipping second one!
Fetching submodule 3rdparty
fatal: remote error: upload-pack: not our ref 03c3817ff[13](https://github.com/nextcloud/server/actions/runs/3475945299/jobs/5812897563#step:7:14)2653c794fd04410977952f69fd6[14](https://github.com/nextcloud/server/actions/runs/3475945299/jobs/5812897563#step:7:15)
Errors during submodule fetch:
	3rdparty

[pserwylo]

Thanks for kicking the builds off (first time contributor so they wont kick off automatically). Now that they've run and we can see that error, I'll tweak then amend the commit, which should kick off a new build, then comment back once it is building as expected.

[pserwylo]

Oh gosh, it needs approval every time I make a change. Let me go try forking my fork, so that I can simulate this without having to bother you all. Will report back when I am more confident in the solution.

[pserwylo]

Okay, have tried this with a fork-of-a-fork requesting a PR to my fork, and it now should work as expected. Code is more explicit and probably what I would do if I was typing the commands out manually (create a new remote, and use that remote when fetching and referring to remote refs).

[pserwylo]

Phew, think we're doing the right thing now, hopefully good to go:

https://github.com/nextcloud/server/actions/runs/3479487910/jobs/5818120859

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

[juliusknorr]

Thanks a lot for that @pserwylo 👍

[nickvergessen]

/backport to stable25