issue: allow configurable default or default to current repo origin

[bmizerany]

Currently, issue defaults to the golang/go repo and there exists no way to change this. This means those using issue and not working on that repo must frequently supply a -p user/repo. I think it would be nice if issue was a little "smarter" about the repo I'm interested in and so I propose adding smarter defaults using the following rules, each overriding the next:

• issue commands use -p if supplied
• issue commands default to git config github.issue.repo.always if supplied
• issue commands default to the origin repo if supplied and its host is github
• issue commands default to git config github.issue.repo if supplied
• issue commands default to golang/go if none of the above are satisfied.

I'm not married to the git config names.

[rsc]

I understand the default is bad for programmers outside the go repos. But in the subrepos we still want golang/go, without additional configuration just for this command. Perhaps instead:

• use $ISSUEREPO if set. otherwise:
• find current repo root
• if $root/codereview.cfg exists and has issuerepo: line, use that
• otherwise if .git/config says origin is github, use that
• otherwise use golang/go

?

[bmizerany]

SGTM! I can work on this change later in the week and send a PR. Should I send PRs to this repo or is there a Gerrit install I should send a CL to?