git-arc: Add patch -b option

With the new patch -b option, switch to a new branch before applying
changes from Differential revisions.

While here, do some minor clean-up:

• Fix a check to determine whether arc patch ran successfully.
• Always ensure at least one argument is supplied to git arc patch.

Sponsored by: The FreeBSD Foundation

Reviewed by: imp, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54024