show-ref can't handle HEAD^

[vlinder]

Would it be possible for the action to switch from show-ref to rev-parse. It works with more ways of specifying commits. Like HEAD^.

git show-ref HEAD^ # fails
git rev-parse HEAD^ # works

Slightly redacted log from merge_group run.

Get current git ref
  /usr/bin/git branch --show-current
  gh-readonly-queue/main/pr-7725-77a9b0
Changes will be detected between HEAD^ and gh-readonly-queue/main/pr-7725-77a9b0
Searching for merge-base HEAD^...gh-readonly-queue/main/pr-7725-77a9b0
  /usr/bin/git show-ref HEAD^
  /usr/bin/git show-ref gh-readonly-queue/main/pr-7725-77a9b0
  9d2a3c[45](https://github.com/<redacted>)ac3313452 refs/heads/gh-readonly-queue/main/pr-7725-77a9b0
  9d2a3c refs/remotes/origin/gh-readonly-queue/main/pr-7725-77a9b0
  /usr/bin/git fetch --no-tags --depth=100 origin HEAD^ gh-readonly-queue/main/pr-7725-77a9b0
  fatal: invalid refspec 'HEAD^'

Using rev-parse would open up use cases like the following:

-      - name: Determine base
-        id: determine_base
-        run: echo "BASE=$(git rev-parse HEAD^1)" >> $GITHUB_OUTPUT
       - uses: dorny/paths-filter
         id: changes
         with:
-          base: ${{ steps.determine_base.outputs.BASE }}
+          base: HEAD^