gpython: '-' means gpython should read program file from stdin

Reported by @jerome here:
nexedi/slapos!862 (comment 121738)

gpython/__init__.py

@@ -157,7 +157,7 @@ def pymain(argv, init=None):
     reexec_argv += argv
     if run is None:
         # file
-        if len(argv) > 0:
+        if len(argv) > 0 and argv[0] != '-':
             sys.argv = argv
             filepath = argv[0]
 
@@ -173,7 +173,7 @@ def pymain(argv, init=None):
 
         # interactive console
         else:
-            sys.argv = ['']
+            sys.argv = ['']  if len(argv) == 0  else  argv # e.g. ['-']
             sys.path.insert(0, '')  # cwd
 
             def run():

gpython/gpython_test.py

@@ -163,6 +163,10 @@ def test_pymain():
     # interactive
     _ = pyout([], stdin=b'import hello\n', cwd=testdata)
     assert _ == b"hello\nworld\n['']\n"
+    _ = pyout(['-'], stdin=b'import hello\n', cwd=testdata)
+    assert _ == b"hello\nworld\n['-']\n"
+    _ = pyout(['-', 'zzz'], stdin=b'import hello\n', cwd=testdata)
+    assert _ == b"hello\nworld\n['-', 'zzz']\n"
 
     # -c <command>
     _ = pyout(['-c', 'import hello', 'abc', 'def'], cwd=testdata)