this CL allows breaking changes to occur (#142)
this CL allows breaking changes to occur (#142)

apidiff currently fails our build when a breaking change is detected.
This needs tweaking, because sometimes we need to allow breaking
changes:

- An alpha|beta client gets a breaking change. This is acceptable
because breaking changes are expected to alpha|beta clients.
- A gRPC service interface gets a breaking change. This is acceptable
because users are not expected to implement gRPC service interfaces.
- A hard decision was made to allow a breaking change for some reason
like a bug fix.

This PR checks for the commit message to contain a
BREAKING_CHANGE_ACCEPTABLE=<reason>.
googleapis/go-genproto,googleapis/go-genproto,google/go-genproto,google/go-genproto