git-am: minor cleanups

git-am: minor cleanups

Update usage statement to remove a no-longer supported option, and to hide two
options (one a no-op, one internal) unless --help-all is used.

Use "test -t 0" instead of "tty -s" to detect when stdin is a terminal. (test
-t 0 is used elsewhere in git-am and in other git shell scripts, tty -s is
not, and appears to be deprecated by POSIX)

Use "test ..." instead of "[ ... ]" and "die <msg>" instead of "echo <msg>
>&2; exit 1" to be consistent with rest of script.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>