fix: suggest to cherry-pick specific commits

The manual instructions still suggested to determine the commits to
cherry-pick by determining the common ancestor (merge-base) of the head
and base refs, first. However, this way has been abandoned for quite
some time now. This especially became a problem now that merge commits
can be skipped, meaning that the suggestion no longer produced a
reliable result.

Instead, we can simply suggest the same as the action does. It knows
exactly which commits to cherry-pick.

src/backport.ts

@@ -53,8 +53,6 @@ export class Backport {
       const repo = payload.repository?.name ?? this.github.getRepo().repo;
       const pull_number = this.github.getPullNumber();
       const mainpr = await this.github.getPullRequest(pull_number);
-      const headref = mainpr.head.sha;
-      const baseref = mainpr.base.sha;
 
       if (!(await this.github.isMerged(mainpr))) {
         const message = "Only merged pull requests can be backported.";
@@ -178,9 +176,8 @@ export class Backport {
           } catch (error) {
             const message = this.composeMessageForCheckoutFailure(
               target,
-              baseref,
-              headref,
               branchname,
+              commitShasToCherryPick,
             );
             console.error(message);
             successByTarget.set(target, false);
@@ -198,9 +195,8 @@ export class Backport {
           } catch (error) {
             const message = this.composeMessageForCherryPickFailure(
               target,
-              baseref,
-              headref,
               branchname,
+              commitShasToCherryPick,
             );
             console.error(message);
             successByTarget.set(target, false);
@@ -380,16 +376,14 @@ export class Backport {
 
   private composeMessageForCheckoutFailure(
     target: string,
-    baseref: string,
-    headref: string,
     branchname: string,
+    commitShasToCherryPick: string[],
   ): string {
     const reason = "because it was unable to create a new branch";
     const suggestion = this.composeSuggestion(
       target,
       branchname,
-      baseref,
-      headref,
+      commitShasToCherryPick,
     );
     return dedent`Backport failed for \`${target}\`, ${reason}.
 
@@ -399,16 +393,14 @@ export class Backport {
 
   private composeMessageForCherryPickFailure(
     target: string,
-    baseref: string,
-    headref: string,
     branchname: string,
+    commitShasToCherryPick: string[],
   ): string {
     const reason = "because it was unable to cherry-pick the commit(s)";
     const suggestion = this.composeSuggestion(
       target,
       branchname,
-      baseref,
-      headref,
+      commitShasToCherryPick,
     );
     return dedent`Backport failed for \`${target}\`, ${reason}.
 
@@ -419,16 +411,14 @@ export class Backport {
   private composeSuggestion(
     target: string,
     branchname: string,
-    baseref: string,
-    headref: string,
+    commitShasToCherryPick: string[],
   ) {
     return dedent`\`\`\`bash
       git fetch origin ${target}
       git worktree add -d .worktree/${branchname} origin/${target}
       cd .worktree/${branchname}
       git checkout -b ${branchname}
-      ancref=$(git merge-base ${baseref} ${headref})
-      git cherry-pick -x $ancref..${headref}
+      git cherry-pick -x ${commitShasToCherryPick.join(" ")}
       \`\`\``;
   }