feat(git): Use --strategy=ours only as fallback

[ChristophWurst]

Fixes #189

This PR changes the merge strategy back to the git default and only falls back to --strategy=ours if the default fails.

The warning in the PR description is now also sensible to the type of conflict:

1. If we fall back to --strategy=ours there is likely a conflict to be resolved
2. If only the diff is not equal between the original PR and the backport PR we don't necessarily have a conflict to resolve. If the default merge strategy succeeded we have a successful backport

In both cases the PR will still be opened as draft.

[nickvergessen]

Nice stuff.

[nickvergessen]

Somehow not convinced:
E.g. here the bot failed:
nextcloud/server#36511 (comment)
But manually cherry-pick is quite fluid and doesn't need manual adjustments 🤔

git cherry-pick bb34476b689cf138f7b394326534291b5b4e8e86
Auto-merging .github/workflows/ftp.yml
Auto-merging .github/workflows/performance.yml
Auto-merging .github/workflows/s3-external.yml
Auto-merging .github/workflows/s3-primary.yml
[backport/36511/stable25 9e8d8854e3f] chore(CI): Opt-out of PHPUnit 10 for now
 Date: Fri Feb 3 10:02:02 2023 +0100
 4 files changed, 5 insertions(+), 5 deletions(-)

[ChristophWurst]

The bot didn't fail the git operation. The branch was created: https://github.com/nextcloud/server/tree/backport/36511/stable25

[ChristophWurst]

#366 is why the PR wasn't created

[nickvergessen]

The branch was created

That was me :)

[ChristophWurst]

good bot