submodule recursing: do not write a config variable twice

submodule recursing: do not write a config variable twice

The command line option for '--recurse-submodules' is implemented
using an OPTION_CALLBACK, which takes both the callback (that sets
the file static global variable) as well as passes the same file
static global variable to the option parsing machinery to assign it.
This is fixed in this commit by passing NULL as the variable. The
callback sets it instead

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>