Parse merge commit recursively

To find a revision from the uploading branch, we find the parent
commit if the commit is a merge commit. But it fails if the parent
commit is also a merge commit. To avoid this, find the parent commit
recursively.

Bug: 156585121
Test: python check_gpl_license.py
Change-Id: I93149e8ee7d3e6b9eb0babdd017dcd9fdb10d4c0
1 file changed