Fix shebang handling for languages with non-# comments

The shebang always starts with the sequence "#!" independend of the
comment symbol of the programming language.
Add a test with Rust, which uses "//" as the comment symbol.

jupytext/header.py

@@ -47,7 +47,7 @@ def encoding_and_executable(notebook, metadata, ext):
     jupytext_metadata = metadata.get('jupytext', {})
 
     if comment is not None and 'executable' in jupytext_metadata:
-        lines.append(comment + '!' + jupytext_metadata.pop('executable'))
+        lines.append('#!' + jupytext_metadata.pop('executable'))
 
     if 'encoding' in jupytext_metadata:
         lines.append(jupytext_metadata.pop('encoding'))
@@ -144,7 +144,7 @@ def header_to_metadata_and_cell(lines, header_prefix, ext=None):
     encoding_re = re.compile(r'^[ \t\f]*{}.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)'.format(comment))
 
     for i, line in enumerate(lines):
-        if i == 0 and line.startswith(comment + '!'):
+        if i == 0 and line.startswith('#!'):
             metadata.setdefault('jupytext', {})['executable'] = line[2:]
             start = i + 1
             continue

tests/test_read_simple_rust.py

@@ -43,3 +43,26 @@ def test_read_simple_file(text='''println!("Hello world");
 ::std::mem::drop''')
     ]))
     compare(jupytext.writes(nb, 'rs'), text)
+
+
+def test_read_write_script_with_metadata_241(no_jupytext_version_number, rsnb="""#!/usr/bin/env scriptisto
+// ---
+// jupyter:
+//   jupytext:
+//     text_representation:
+//       extension: .rs
+//       format_name: light
+//   kernelspec:
+//     display_name: Rust
+//     language: rust
+//     name: rust
+// ---
+
+let mut a: i32 = 2;
+a += 1;
+"""):
+    nb = jupytext.reads(rsnb, 'rs')
+    assert 'executable' in nb.metadata['jupytext']
+    rsnb2 = jupytext.writes(nb, 'rs')
+
+    compare(rsnb, rsnb2)