Prevent failures when files referenced in .dsc are missing

Only mark existent remote files as superseded, as otherwise we’ll not be able to compare them with the local files.

This fixes failures like this.

osc_plugin_dput/main.py

@@ -191,7 +191,6 @@ def do_dput(self, subcmd, opts, *args):
         # Remove old files, but only those that are part of the Debian package
         superseded = set()
         retained = set()
-        downloaded = set()
 
         for f in remote_file_list.keys():
             if f.endswith('.dsc'):
@@ -205,7 +204,8 @@ def do_dput(self, subcmd, opts, *args):
                 remote_dsc = Dsc(osc.core.streamfile(u, bufsize='line'))
 
                 for entry in remote_dsc.get('Files'):
-                    superseded.add(entry['name'])
+                    if entry['name'] in remote_file_list:
+                        superseded.add(entry['name'])
 
                 superseded.add(f)
             elif f.endswith('.changes'):
@@ -232,7 +232,7 @@ def do_dput(self, subcmd, opts, *args):
         os.symlink(dsc_file, f)
         package.addfile(f)
 
-        if opts.maintained_in_git and 'MAINTAINED_IN_GIT.txt' not in retained:
+        if opts.maintained_in_git:
             open('MAINTAINED_IN_GIT.txt', 'w+').close()
             package.addfile('MAINTAINED_IN_GIT.txt')
 
@@ -241,9 +241,6 @@ def do_dput(self, subcmd, opts, *args):
         # the source package
         msg = f
 
-        for f in downloaded:
-            package.addfile(f)
-
         if changes is not None:
             f = dsc_or_changes_file.name
             os.symlink(dsc_or_changes_file, f)