The GerritProject.GitCommit method in x/build/maintner returns an error
as of https://go.dev/cl/407394. Start using its new API.

It has changed in https://github.com/sourcegraph/go-diff/pull/30.

Updates https://github.com/sourcegraph/go-diff/issues/27

Also remove commented out sort code. It's normal and expected for
Gerrit IDs not to line up with created times. As documented at
https://godoc.org/golang.org/x/build/maintner#GerritCL.Number:

> Number is the CL number on the Gerrit server (e.g. 1, 2, 3). Gerrit CL
> numbers are sparse (CL N does not guarantee that CL N-1 exists) and
> Gerrit issues CL's out of order - it may issue CL N, then CL (N - 18),
> then CL (N - 40).

A CL is considered to not exist if it's private or if it has an empty
status (possible due to an upstream bug).

Updates https://golang.org/issue/22060.

Add unparsed messages as simple comments. This way, value of Replies
corresponds to actual number of messages.

Update to current changes.Service interface.

Rather than "not found", which is misleading and makes it look like the
implementation isn't working (when in fact there is no implementation).

Simplify some code by indenting the uncommon path.

Update docs to be more accurate.