Fix misdetection of project root with --stdin-filename

There are a number of places this behaviour could be patched, for
instance, it's quite tempting to patch it in `get_sources`. However
I believe we generally have the invariant that project root contains all
files we want to format, in which case it seems prudent to keep that
invariant.

Fixes psf#3207

src/black/__init__.py

@@ -468,7 +468,13 @@ def main(  # noqa: C901
         out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
         ctx.exit(1)
 
-    root, method = find_project_root(src) if code is None else (None, None)
+    if stdin_filename is not None:
+        src_with_stdin_filename = tuple(stdin_filename if s == "-" else s for s in src)
+    else:
+        src_with_stdin_filename = src
+    root, method = (
+        find_project_root(src_with_stdin_filename) if code is None else (None, None)
+    )
     ctx.obj["root"] = root
 
     if verbose: