git-rebase: report checkout failure

git-rebase: report checkout failure

When detaching the HEAD to the base commit, the "git checkout" command
could fail if, for example, upstream contains a file that would overrwrite
a local, untracked file.  Unconditionally discarding the standard error
stream was done to squelch the progress and notices back when checkout
did not have -q option, but there is no reason to keep doing it anymore.

Noticed by Robert Shearman.

Signed-off-by: Junio C Hamano <gitster@pobox.com>