add: make pathless 'add [-u|-A]' warning a file-global function

add: make pathless 'add [-u|-A]' warning a file-global function

Currently warn_pathless_add() is only called directly by cmd_add(),
but that is about to change.  Move its definition higher in the file
and pass the "--update" or "--all" option name used in its message
through globals instead of function arguments to make it easier to
call without passing values that will not change through the call
chain.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>