packaging: write out editor.rc in Python

Let's dynamically generate the file from Python instead of
relying on Inno functionality, as that will give more
visibility into what's going on.

I spotted this as part of converging the install layout
of Inno and WiX: the former doesn't install an editor.rc
and instead defines the editor via its mercurial.rc file.

We will change the behavior of WiX later once it is consuming
the staged install directory for packaging.

Differential Revision: https://phab.mercurial-scm.org/D7171

contrib/packaging/hgpackaging/py2exe.py

@@ -217,6 +217,13 b' def stage_install(source_dir: pathlib.Pa'
     """
     process_install_rules(STAGING_RULES, source_dir, staging_dir)
 
+    # Write out a default editor.rc file to configure notepad as the
+    # default editor.
+    with (staging_dir / 'hgrc.d' / 'editor.rc').open(
+        'w', encoding='utf-8'
+    ) as fh:
+        fh.write('[ui]\neditor = notepad\n')
+
     # Purge any files we don't want to be there.
     for f in STAGING_EXCLUDES:
         p = staging_dir / f

contrib/packaging/inno/mercurial.iss

@@ -48,7 +48,6 b' Source: {{ entry.source }}; DestDir: {{ '
 
 [INI]
 Filename: {app}\Mercurial.url; Section: InternetShortcut; Key: URL; String: https://mercurial-scm.org/
-Filename: {app}\hgrc.d\editor.rc; Section: ui; Key: editor; String: notepad
 
 [UninstallDelete]
 Type: files; Name: {app}\Mercurial.url