Add branch-sha input

prepare-release/action.yml

@@ -7,6 +7,8 @@ inputs:
   branch:
     description: The target branch to use for the release.
     default: ${{ github.event.repository.default_branch }}
+  branch-sha:
+    description: The SHA from which to create the target branch if missing.
   changelog-author:
     description: Git-format author to use for the changelog commits.
     default: Conda Bot <[email protected]>
@@ -15,7 +17,7 @@ inputs:
     default: ${{ github.token }}
   fork-token:
     description: 'GitHub token to create and push to the fork. Fine-grained PAT: `administration: write; contents: write`'
-    default: ${{ github.token }}
+    default: ${{ inputs.token }}
 runs:
   using: composite
   steps:
@@ -34,7 +36,7 @@ runs:
             -H "Accept: application/vnd.github+json" \
             "/repos/${{ github.repository }}/git/refs" \
             -f ref="refs/heads/${{ inputs.branch }}" \
-            -f sha="$(git rev-parse ${{ github.event.repository.default_branch }})"
+            -f sha="$(git rev-parse ${{ inputs.branch-sha || github.event.repository.default_branch }})"
         fi
         git fetch origin
         git checkout "${{ inputs.branch }}"