workflows: run on merge conflicts as well

When a PR is having conflicts with the base branch, we used to skip most
jobs depending on the target branch. With this change, we still run
those jobs, but without actually merging the PR temporarily. That means
we compare the head of the PR with the merge-base of the PR's branch and
the target branch - i.e. the point where the PR branched off.

This is not 100% accurate, but that's not important, because after
resolving the merge conflicts, those workflows will run again anyway. It
allows to give early feedback, though, instead of just skipping all the
jobs.

.github/workflows/get-merge-commit.yml

@@ -68,7 +68,18 @@ jobs:
 
                 return { mergedSha, targetSha }
               } else {
-                throw new Error("The PR has a merge conflict. Skipping the rest...")
+                console.log("The PR has a merge conflict.")
+
+                const mergedSha = prInfo.head.sha
+                const targetSha = (await github.rest.repos.compareCommitsWithBasehead({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  basehead: `${prInfo.base.sha}...${prInfo.head.sha}`
+                })).data.merge_base_commit.sha
+
+                console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
+
+                return { mergedSha, targetSha }
               }
             }
             throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")