Check parent commit if revision is from *-release

Revisions in a *-release branch include merge CLs that merge
*-dev to *-release. These revisions will not be in the source
of a GPL git project, so in these cases need to extract the parent
revision of the merge commit.

Test: python check_gpl_license.py --branch pi-release --build
<bid> 27 -vv
Bug: 78279738
Change-Id: I2950b56605b9af39c3ab012ae7a1ad407942d419