Ensure display name changes are logged

https://github.com/python-discord/bot/issues/489

Recently, we discovered that not all display name changes were logged
to the #user-log channel. This problem was caused by the `old_value`
or the `new_value` showing up as `None` when a user sets or removes a
guild-specific nickname. Since we ignore changes where one of the two
values is `None`, we did not log these `None->nick` or `nick->None`
events.

Since we are mainly interested in the display name of the user, and
the display name is equal to the user's guild-specific nickname if
they have set one and otherwise their username, I made the following
changes:

- Add logging of changes in the display names of members.

- Ignore nick-specific changes completely, since these changes are
  already captured by the changes in the display name we now log.

This closes #489

1 files changed, 6 insertions, 1 deletions

diff --git a/bot/cogs/moderation/modlog.py b/bot/cogs/moderation/modlog.py
index 86eab55de..92e9b0ef1 100644
--- a/bot/cogs/moderation/modlog.py
+++ b/bot/cogs/moderation/modlog.py
@@ -20,7 +20,7 @@ GUILD_CHANNEL = t.Union[discord.CategoryChannel, discord.TextChannel, discord.Vo
 
 CHANNEL_CHANGES_UNSUPPORTED = ("permissions",)
 CHANNEL_CHANGES_SUPPRESSED = ("_overwrites", "position")
-MEMBER_CHANGES_SUPPRESSED = ("status", "activities", "_client_status")
+MEMBER_CHANGES_SUPPRESSED = ("status", "activities", "_client_status", "nick")
 ROLE_CHANGES_UNSUPPORTED = ("colour", "permissions")
 
 
@@ -498,6 +498,11 @@ class ModLog(Cog, name="ModLog"):
                 f"**Discriminator:** `{before.discriminator}` **->** `{after.discriminator}`"
             )
 
+        if before.display_name != after.display_name:
+            changes.append(
+                f"**Display name:** `{before.display_name}` **->** `{after.display_name}`"
+            )
+
         if not changes:
             return