Improve show buffer numbers on "/buffer" command. Checks completion
against list of commands to avoid ambigious "/b" matching ("BUFFER" v.
"BAN"). Matches uppercase and mixed case i.e. "/BU" or "/bU".
I see, thanks for the patch. While I don't use neither buffer numbers
nor /buffer myself, I expect typing "/b <number>" be the common
practice, and this patch breaks it? What do you think?