fix: checkout target branch before making changes

The previous implementation was creating PRs with the entire diff between
the default branch and target branch. Now we properly checkout the target
branch first, then make dependency updates on top of it.

This ensures PRs only contain the dependency changes, not all differences
between branches.

Author: vaind

updater/action.yml

@@ -109,6 +109,13 @@ runs:
       with:
         token: ${{ inputs.api-token }}
 
+    - name: Checkout target branch
+      if: inputs.target-branch != ''
+      shell: bash
+      run: |
+        git fetch origin ${{ inputs.target-branch }}:${{ inputs.target-branch }}
+        git checkout ${{ inputs.target-branch }}
+
     - name: Update to the latest version
       id: target
       shell: pwsh
@@ -235,6 +242,13 @@ runs:
       with:
         token: ${{ inputs.api-token }}
 
+    - name: 'After new PR: checkout target branch'
+      if: ${{ inputs.target-branch != '' && ( steps.target.outputs.latestTag != steps.target.outputs.originalTag ) && ( steps.existing-pr.outputs.url == '') && ( steps.root.outputs.changed == 'false') }}
+      shell: bash
+      run: |
+        git fetch origin ${{ inputs.target-branch }}:${{ inputs.target-branch }}
+        git checkout ${{ inputs.target-branch }}
+
     - name: 'After new PR: redo the update'
       if: ${{ ( steps.target.outputs.latestTag != steps.target.outputs.originalTag ) && ( steps.existing-pr.outputs.url == '') && ( steps.root.outputs.changed == 'false') }}
       shell: pwsh