Merge branch 'jk/am-retry'

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