pathspec: create parse_long_magic function

pathspec: create parse_long_magic function

Factor out the logic responsible for parsing long magic into its own
function.  As well as hoist the prefix check logic outside of the inner
loop as there isn't anything that needs to be done after matching
"prefix:".

Signed-off-by: Brandon Williams <redacted>
Reviewed-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>