terminal: don't assume stdin is /dev/tty

read_key_without_echo() reads from stdin but uses /dev/tty when it
disables echo. This is unfortunate as there no guarantee that stdin is
the same device as /dev/tty. The perl version of "add -p" uses stdin
when it sets the terminal mode, this commit does the same for the
builtin version. There is still a difference between the perl and
builtin versions though - the perl version will ignore any errors when
setting the terminal mode[1] and will still read single bytes when
stdin is not a terminal. The builtin version displays a warning if
setting the terminal mode fails and switches to reading a line at a
time.

[1] b061c913bb/ReadKey.xs (L1090)

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/terminal.h

@@ -4,6 +4,8 @@
 enum save_term_flags {
 	/* Save input and output settings */
 	SAVE_TERM_DUPLEX = 1 << 0,
+	/* Save stdin rather than /dev/tty (fails if stdin is not a terminal) */
+	SAVE_TERM_STDIN  = 1 << 1,
 };
 
 /*