scripts: Adopted default visibility in scripts with complex fields

This makes it so scripts with complex fields will still output all
fields to output csv/json files, while only showing a user-friendly
subset unless -f/--field is explicitly provided.

While internal fields are often too much information to show by default,
csv/json files are expected to go to other scripts, not humans. So more
information is more useful up until you actually hit a performance
bottleneck.

And if you _do_ somehow manage to hit a performance bottleneck, you can
always limit the output with explicit -f/--field flags.

scripts/cov.py

@@ -953,11 +953,13 @@ def main(gcda_paths, *,
         else:
             by = ['function']
 
+    visible = None
     if fields is None:
+        fields = ['calls', 'hits', 'funcs', 'lines', 'branches']
         if not hits:
-            fields = ['lines', 'branches']
+            visible = ['lines', 'branches']
         else:
-            fields = ['calls', 'hits']
+            visible = ['calls', 'hits']
 
     # find sizes
     if not args.get('use', None):
@@ -1019,7 +1021,7 @@ def main(gcda_paths, *,
             # print table
             table(CovResult, results, diff_results,
                     by=by,
-                    fields=fields,
+                    fields=visible if visible is not None else fields,
                     sort=sort,
                     **args)