Expand links when checking out a single file.

Prior to this change, `osc co PROJECT' and `osc co PROJECT PACKAGE'
would both follow links, but `osc co PROJECT PACKAGE FILE' would not.

To make matters even more confusing, `osc ls PROJECT PACKAGE FILE'
does follow links - so, for linked files, an `ls' would tell you a file
exists, then `co' would give a 404 error.

This change fixes the inconsistency.

osc/commandline.py

@@ -2886,6 +2886,22 @@ Please submit there instead, or use --nodevelproject to force direct submission.
             sys.exit(1)
 
         if filename:
+            link_seen = False
+            # Note: this logic should follow the `osc ls' logic
+            if expand_link:
+                l = meta_get_filelist(apiurl,
+                                  project,
+                                  package,
+                                  expand=False,
+                                  revision=rev)
+                link_seen = '_link' in l
+            if link_seen:
+                m = show_files_meta(apiurl, project, package)
+                li = Linkinfo()
+                li.read(ET.fromstring(''.join(m)).find('linkinfo'))
+                if li.haserror():
+                    raise oscerr.LinkExpandError(project, package, li.error)
+                project, package, rev = li.project, li.package, li.rev
             get_source_file(apiurl, project, package, filename, revision=rev, progress_obj=self.download_progress)
 
         elif package: