Value of argument "first-parent" causing error [fatal: ambiguous argument]

When execute gitleaks in GitHub Actions pipeline triggered by Pull Request in main / master branch, there will be an argument "first-parent" with a long hash / string.

gitleaks detect --redact -v --exit-code=2 --report-format=sarif --report-path=results.sarif --log-level=debug --log-opts=--no-merges --first-parent 285152a60a0fc48c328c090a59e73dee6732386c^..e03be20820e328exxxxxxxxf2cbfc

This argument does not exist when push to non master / main branch, or just push code to any branch. It only exists when merge to main / master. But it will fail with following reason

ERR [git] fatal: ambiguous argument '285152a60a0fc48c328c090a59e73dee6732386c^..e03be20820e328exxxxxxxxf2cbfc': unknown revision or path not in the working tree.

May I ask how this can be resolved.

Thank you.

Here are the log:

`Sent 2682970 of 2682970 (100.0%), 22.2 MBs/sec
event type: pull_request
gitleaks cmd: gitleaks detect --redact -v --exit-code=2 --report-format=sarif --report-path=results.sarif --log-level=debug --log-opts=--no-merges --first-parent 285152a60a0fc48c328c090a59e73dee6732386c^..e03be20820e328exxxxxxxxf2cbfc
/tmp/gitleaks-8.16.1/gitleaks detect --redact -v --exit-code=2 --report-format=sarif --report-path=results.sarif --log-level=debug --log-opts=--no-merges --first-parent 285152a60a0fc48c328c090a59e73dee6732386c^..e03be20820e328exxxxxxxxf2cbfc

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░ gitleaks

6:51AM DBG using gitleaks config from GITLEAKS_CONFIG env var: ./gitleaks.toml
6:51AM DBG executing: /usr/bin/git -C . log -p -U0 --no-merges --first-parent 285152a60a0fc48c328c090a59e73dee6732386c^..e03be20820e328exxxxxxxxf2cbfc
6:51AM ERR [git] fatal: ambiguous argument '285152a60a0fc48c328c090a59e73dee6732386c^..e03be20820e328exxxxxxxxf2cbfc': unknown revision or path not in the working tree.
6:51AM ERR [git] Use '--' to separate paths from revisions, like this:
6:51AM ERR [git] 'git [...] -- [...]'
6:51AM INF 0 commits scanned.
6:51AM DBG Note: this number might be smaller than expected due to commits with no additions
6:51AM ERR error="git error encountered, see logs"
6:51AM WRN partial scan completed in 63.9ms
6:51AM WRN no leaks found in partial scan`

Start at the GitHub Actions invocation that builds the gitleaks command, then reproduce the pull_request case using the logged revision range and compare it with a branch push. Trace how the action obtains the commits and determine what successful behavior should be when scanning the pull request without a Git ambiguous-argument error.

Written by the indexing model from the issue text.