[commands] Ensure that Context.command is the command in Command.can_run

Previously, Context.command was not guaranteed to be the actual command
being checked if it can run. This could be troublesome when
implementing help commands or when using the default help command.

This new change allows at least for the guarantee that Context.command
to be technically correct in Command.can_run.

discord/ext/commands/core.py

@@ -640,6 +640,10 @@ class Command:
             A boolean indicating if the command can be invoked.
         """
 
+        original = ctx.command
+        ctx.command = self
+
+        try:
             if not (yield from ctx.bot.can_run(ctx)):
                 raise CheckFailure('The global check functions for command {0.qualified_name} failed.'.format(self))
 
@@ -660,6 +664,8 @@ class Command:
                 return True
 
             return (yield from discord.utils.async_all(predicate(ctx) for predicate in predicates))
+        finally:
+            ctx.command = original
 
 class GroupMixin:
     """A mixin that implements common functionality for classes that behave