git-codereview: use codereview.cfg from working tree

Loading codereview.cfg from origin/master
effictively means that users have to ask permission
to use git-codereview.
With this change, users can opt to add the file,
.gitignore it, and forget about it.

Looking in origin/master also makes it hard
to try out git-codereview on a project
without having to commit to it.

Instead of requiring codereview.cfg be checked in
anywhere, just look on the filesystem for it.
I can't figure out why I ever thought
doing otherwise was a good idea.

There are some other related fixes floating around,
which might also be good to put in,
but this is seems like a good stop-gap,
since it is a minimal change;
codereview.cfg continues to work for anyone
currently using it.

Related: golang/go#15616
Related: golang/go#15073

Change-Id: I1e377819f8eb8c8fecf9f022459551a3e8b93d48
Reviewed-on: https://go-review.googlesource.com/24001
Reviewed-by: Andrew Gerrand <adg@golang.org>
3 files changed