checkout: print something when checking out paths

checkout: print something when checking out paths

One of the problems with "git checkout" is that it does so many
different things and could confuse people specially when we fail to
handle ambiguation correctly.

One way to help with that is tell the user what sort of operation is
actually carried out. When switching branches, we always print
something unless --quiet, either

 - "HEAD is now at ..."
 - "Reset branch ..."
 - "Already on ..."
 - "Switched to and reset ..."
 - "Switched to a new branch ..."
 - "Switched to branch ..."

Checking out paths however is silent. Print something so that if we
got the user intention wrong, they won't waste too much time to find
that out. For the remaining cases of checkout we now print either

 - "Checked out ... paths out of the index"
 - "Checked out ... paths out of <abbrev hash>"

Since the purpose of printing this is to help disambiguate. Only do it
when "--" is missing.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>