scripts: Renamed -p/--percent -> -%/--percent

How long has there been a whole key dedicated to percentages sitting on
my keyboard!?

There's some funky business with format strings in argparse, but this
was already worked around for dbgbmap.py's -%/--usage flag.

scripts/perfbd.py

@@ -1690,9 +1690,13 @@ if __name__ == "__main__":
     parser.add_argument(
             '-d', '--diff',
             help="Specify CSV/JSON file to diff against.")
+    # need a special Action here because this % causes problems
+    class StoreTruePercent(argparse._StoreTrueAction):
+        def format_usage(self):
+            return '-%%'
     parser.add_argument(
-            '-p', '--percent',
-            action='store_true',
+            '-%', '--percent',
+            action=StoreTruePercent,
             help="Only show percentage change, not a full diff.")
     parser.add_argument(
             '-C', '--compare',