Don't try to default `target_commitish` to the currently checked out
branch. Instead, keep the value blank unless explicitly provided. It
will default to the main branch on the server.

Fixes #446, fixes #937

Closes #1145, fixes #1118

Fixes #1069, fixes #1206, fixes #1133, fixes #1236

In case when the PR's head and base repositories are the same, check out
the new branch in a way that sets up push workflow to the same branch.

    git fetch origin +refs/heads/fixes:refs/remotes/origin/fixes
    git checkout -b fixes --track origin/fixes

The remote needs to match the base project from the pull request.

Graduate from beta

Revamp `hub issue` commands

Instead, specify timezone offsets explicitly so that different
environments interpret the time identically.