maintner: add GitCommit.HasAncestor method

maintnerd will have an API to query this in a subsequent CL.

Updates golang/go#20222

Change-Id: I3cb0ce2897eea782f627c2e27c4d02ef0eb6c66b
Reviewed-on: https://go-review.googlesource.com/43554
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed