[commands] Restart the StringView properly in Context.reinvoke.
[commands] Restart the StringView properly in Context.reinvoke.

The old code for some reason assumed that the length of the command
would be 1. This is because when I tested this I would use single
letter command names and it would "just work" when in reality it was
completely incorrect.

A reminder to thoroughly test instead of just fitting something to work

mgardne8/discord.py,khazhyk/discord.py,Harmon758/discord.py,Harmon758/discord.py,imayhaveborkedit/discord.py,rapptz/discord.py,Rapptz/discord.py