merge-recursive: porcelain messages for checkout

merge-recursive: porcelain messages for checkout

A porcelain message was first added in checkout.c in the commit
8ccba008 (Junio C Hamano, Sat May 17 21:03:49 2008, unpack-trees:
allow Porcelain to give different error messages) to give better feedback
in the case of merge errors.

This patch adapts the porcelain messages for the case of checkout
instead. This way, when having a checkout error, "merge" no longer
appears in the error message.

While we're there, we add an advice in the case of
would_lose_untracked_file.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>