checkout: split off a function to peel away branchname arg

checkout: split off a function to peel away branchname arg

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>