git checkout can be used to switch branches and to retrieve files from
the index or an arbitrary tree. Split the description into
subsections corresponding to each mode to make each use easier to
understand.
Helped-by: Jeff King <peff@peff.net> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>