The code to parse and consume the tree name and "--" in commands such
as "git checkout @{-1} -- '*.c'" is intimidatingly long. Split it out
into a separate function and make it easier to skip on first reading
by making the data it uses and produces more explicit.
No functional change intended.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>