We no longer guess the head repository using heuristics; instead, we
present the user with the choice of pushable repositories and an
additional option to create a new fork.
The new `pr create --head` flag is available for the user to specify the
head branch in `branch` or `owner:branch` format and completely skip any
forking or auto-pushing checks.
On first run in a git repository, `BaseRepo()` will now prompt the user
which repository should be queried as base repository if there are
multiple git remotes or when we are in the context of a fork.
In non-interactive mode, the prompt is skipped and we default to the
first git remote instead.
After the base repo is resolved, the result is cached in the local
repository using `git config` so that RepositoryNetwork API lookups can
be avoided in the future.
When applying metadata to the new PR such as assignees or reviewers, if
the operation fails, an error message would get printed:
failed to create pull request: <API error text>
This was misleading, because the PR did get created; it's just that
updating it failed. The new error message is:
https://github.com/OWNER/REPO/pull/123
pull request update failed: <API error text>
The PR URL is printed on stdout and the error message is printed on
stderr. In case of any errors, the exit code is still non-zero.
