bisect: improve error message when branch checkout fails

bisect: improve error message when branch checkout fails

In "git-bisect.sh" the "git checkout" command is only used to
change the current branch, but it is used like this:

git checkout "$branch"

which will output the following misleading error message when
it fails:

error: pathspec 'foo' did not match any file(s) known to git.

This patch change the way we use "git checkout" like this:

git checkout "$branch" --

so that we will get the following error message:

fatal: invalid reference: foo

which is better.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>