test: add a line

[korthout]

This will test the default mainline functionality in https://github.com/korthout/backport-action/tree/341-default-mainline

See korthout/backport-action#341

[korthout]

/backport

[github-actions]

Backport failed for case4-target-branches, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin case4-target-branches
git worktree add -d .worktree/backport-266-to-case4-target-branches origin/case4-target-branches
cd .worktree/backport-266-to-case4-target-branches
git checkout -b backport-266-to-case4-target-branches
ancref=$(git merge-base 754e914ec3a9bdb7cbbc45a586074b5df66799f1 f2230426140cf8fc4c410f68d14a8410ce420ac5)
git cherry-pick -x $ancref..f2230426140cf8fc4c410f68d14a8410ce420ac5

[korthout]

So, the behavior is as before (the above ran without a default_mainline). The next run will have default_mainline: 1.

/backport

[github-actions]

Successfully created backport PR for case4-target-branches:

• [Backport case4-target-branches] test: add a line #267

[korthout]

Okay, so that worked. Let's see how it runs with default_mainline: 2

/backport

[github-actions]

Backport failed for case4-target-branches, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin case4-target-branches
git worktree add -d .worktree/backport-266-to-case4-target-branches origin/case4-target-branches
cd .worktree/backport-266-to-case4-target-branches
git checkout -b backport-266-to-case4-target-branches
ancref=$(git merge-base 754e914ec3a9bdb7cbbc45a586074b5df66799f1 f2230426140cf8fc4c410f68d14a8410ce420ac5)
git cherry-pick -x $ancref..f2230426140cf8fc4c410f68d14a8410ce420ac5

[korthout]

Okay, that did not work because the mainline is applied to all cherry-picks:

git cherry-pick -x --mainline=2 837d73b f223042
error: commit 837d73b does not have parent 2

So, default_mainlines of >1 are not supported at this time. It may be useful to add in the future, that would mean that we'd need to use the mainline setting only when cherry-picking merge commits explicitly. This can be added later while keeping behavior the same.

[korthout]

Another idea is to allow specifying 0 as the mainline, where merge commits are skipped. This can also be supported later.