Size reports: Fix stale-commit determination (#11483)
Size reports: Fix stale-commit determination (#11483)

#### Problem

Some size reports are missing. The reason is that `gh_report.py` uses
the commit ‘committer’ time to determine which commits are outdated,
and a rebase sets that to the same value for all commits in a PR.

#### Change overview

Use the ‘author’ timestamp as a secondary sort key.

#### Testing

Manual run confirming sort order.
nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip