Since geiser-guile--startup is calling in the new buffer already, buffer-local
value of geiser-guile-warning-level had no effect.  Luckily the original buffer
is accessible using geiser-repl--last-scm-buffer, so we can let-bind the value
from it for the duration of geiser-guile-update-warning-level.

Fixes #45.

* geiser-guile.el (geiser-guile--startup): Let-bind geiser-guile-warning-level
before calling geiser-guile-update-warning-level.