feat(backport): exclude headref from found target branches

This case is relevant where the target_branches input is used to select
the target branches, and the original pull request's headref is part of
it.

For example, consider selecting all feature branches `feature/*` as
target_branches. Glob patterns are not supported yet, but users can find
these with an additional step in their workflow:

```
run: |
  branches=$(git branch --list --all | grep 'origin/feature/' | cut -c 18- )
  space_delimited=${branches//$'\n'/ }
  echo "BRANCHES=${space_delimited}" >> $GITHUB_OUTPUT
```

Note that such a step requires a deep checkout `depth=0`.

But when users do this, they might encounter the feature branch they
just merged, the merged branch that triggered the backport-action.

By excluding it now, we can better support this case. In addition, it
will help support glob patterns in the target_branches input in the
future.

(cherry picked from commit 151736d)

src/backport.ts

@@ -378,10 +378,13 @@ export function findTargetBranches(
   console.log(
     `Found target branches in \`target_branches\` input: ${configuredTargetBranches}`
   );
+  console.log(
+    `Exclude pull request's headref from target branches: ${headref}`
+  );
 
   const targetBranches = [
     ...new Set([...targetBranchesFromLabels, ...configuredTargetBranches]),
-  ];
+  ].filter((t) => t !== headref);
 
   console.log(`Determined target branches: ${targetBranches}`);
 

src/test/backport.test.ts

@@ -57,6 +57,26 @@ describe("find target branches", () => {
         )
       ).toEqual([]);
     });
+
+    it("when the label pattern only matches the headref", () => {
+      expect(
+        findTargetBranches(
+          { labels: { pattern: default_pattern } },
+          ["backport feature/one"],
+          "feature/one"
+        )
+      ).toEqual([]);
+    });
+
+    it("when target_branches only contains the headref", () => {
+      expect(
+        findTargetBranches(
+          { labels: {}, target_branches: "feature/one" },
+          [],
+          "feature/one"
+        )
+      ).toEqual([]);
+    });
   });
 
   describe("returns selected branches", () => {
@@ -118,5 +138,25 @@ describe("find target branches", () => {
         )
       ).toEqual(["release-1"]);
     });
+
+    it("when several labels match the pattern the headref is excluded", () => {
+      expect(
+        findTargetBranches(
+          { labels: { pattern: default_pattern } },
+          ["backport feature/one", "backport feature/two"],
+          "feature/one"
+        )
+      ).toEqual(["feature/two"]);
+    });
+
+    it("when several target branches are specified the headref is excluded", () => {
+      expect(
+        findTargetBranches(
+          { labels: {}, target_branches: "feature/one feature/two" },
+          [],
+          "feature/one"
+        )
+      ).toEqual(["feature/two"]);
+    });
   });
 });