Also skip unmodified fields in the regex checks

Now it's close to 1s.

fdroidserver/metadata.py

@@ -484,11 +484,15 @@ valuetypes = {
 # Check an app's metadata information for integrity errors
 def check_metadata(app):
     for v in valuetypes:
-        for f in v.fields:
-            v.check(app.get_field(f), app.id)
+        for k in v.fields:
+            if k not in app._modified:
+                continue
+            v.check(app.__dict__[k], app.id)
         for build in app.builds:
-            for f in v.flags:
-                v.check(build.get_flag(f), app.id)
+            for k in v.flags:
+                if k not in build._modified:
+                    continue
+                v.check(build.__dict__[k], app.id)
 
 
 # Formatter for descriptions. Create an instance, and call parseline() with