Merge branch 'pt/am-builtin-options'

After "git am --opt1" stops, running "git am --opt2" pays attention
to "--opt2" only for the patch that caused the original invocation
to stop.

* pt/am-builtin-options:
am: let --signoff override --no-signoff
am: let command-line options override saved options
test_terminal: redirect child process' stdin to a pty