source: Actually check if stdin is a tty, not just redirected

This broke fishtape, which did

    somestuff | fish -c "source"

Because `source` didn't have a redirection, it refused to read from
stdin.

So, to keep the common issue of `source (command that does not print)`
from seeminly stopping fish, we instead actually check if stdin is a terminal.

src/builtin_source.cpp

@@ -41,7 +41,7 @@ int builtin_source(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
 
     if (argc == optind || wcscmp(argv[optind], L"-") == 0) {
         // Either a bare `source` which means to implicitly read from stdin or an explicit `-`.
-        if (argc == optind && !streams.stdin_is_directly_redirected) {
+        if (argc == optind && isatty(streams.stdin_fd)) {
             // Don't implicitly read from the terminal.
             return STATUS_CMD_ERROR;
         }

tests/test1.in

@@ -214,7 +214,7 @@ echo 'echo "source argv {$argv}"' | source -
 echo 'echo "source argv {$argv}"' | source - abc
 echo 'echo "source argv {$argv}"' | source - abc def
 # This hangs if it fails!
-source
+source </dev/tty
 echo $status
 
 always_fails