maintner/cmd/maintserve: don't count PRs as issues
maintner/cmd/maintserve: don't count PRs as issues

Pull requests are now in the maintner corpus via *maintner.GitHubIssue.
We don't want to count them as issues, since they're not issues in a
strict sense.

Follows https://golang.org/cl/69590
and https://github.com/shurcooL/issues/commit/ecd998cade46f29d5d157baff740745f1b36f915.

Change-Id: Ia89e89e263626e661633f888c24070073f6fe567
Reviewed-on: https://go-review.googlesource.com/97797
Run-TryBot: Dmitri Shuralyov <3e7b0c396ed8de245aa9bebbd359a431b4980612@shuralyov.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build