am/apply: warn if we end up reading patches from terminal

In an interactive session, "git am" without arguments, or even
worse, "git am --whitespace file", waits silently for the user to
feed the patches from the standard input (presumably by typing or
copy-pasting).  Give a feedback message to the user when this
happens, as it is unlikely that the user meant to do so.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mailsplit.c

@@ -223,6 +223,9 @@ static int split_mbox(const char *file, const char *dir, int allow_bare,
 	FILE *f = !strcmp(file, "-") ? stdin : fopen(file, "r");
 	int file_done = 0;
 
+	if (isatty(fileno(f)))
+		warning(_("reading patches from stdin/tty..."));
+
 	if (!f) {
 		error_errno("cannot open mbox %s", file);
 		goto out;