Checking upstream does not respect -b

[rhysd]

When -b is set, upstream should be detected based on the branch. But current branch is used always.

Repro:

$ git checkout -b tmp
$ git brws -b master

Expected:
It can open a page for master branch

Actual:
It causes following error since tmp does not set upstream remote branch.

Git command exited with non-zero status: fatal: no upstream configured for branch 'tmp'