Merge branch 'jk/am-retry'

"git am" has a safety feature to prevent it from starting a new session when there already is a session going. It reliably triggers when a mbox is given on the command line, but it has to rely on the tty-ness of the standard input. Add an explicit way to opt out of this safety with a command line option. * jk/am-retry: test-terminal: drop stdin handling am: add explicit "--retry" option