parse-options: free previous value of `OPTION_FILENAME`

parse-options: free previous value of `OPTION_FILENAME`

The `OPTION_FILENAME` option always assigns either an allocated string
or `NULL` to the value. In case it is passed multiple times it does not
know to free the previous value though, which causes a memory leak.

Refactor the function to always free the previous value. None of the
sites where this option is used pass a string constant, so this change
is safe.

While at it, fix the argument of `fix_filename()` to be a string
constant. The only reason why it's not is because we use it as an
in-out-parameter, where the input is a constant and the output is not.
This is weird and unnecessary, as we can just return the result instead
of using the parameter for this.

This leak is being hit in t7621, but plugging it alone does not make the
test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>