config: do not leak excludes_file

config: do not leak excludes_file

The excludes_file variable is marked "const char *", but all the
assignments to it are made with a piece of memory allocated just
for it, and the variable is responsible for owning it.

When "core.excludesfile" is read, the code just lost the previous
value, leaking memory.  Plug it.

The real problem is that the variable is mistyped; our convention
is to never make a variable that owns the piece of memory pointed
by it as "const".  Fixing that would reduce the chance of this kind
of bug happening, and also would make it unnecessary to cast the
constness away while free()ing it, but that would be a much larger
follow-up effort.

Reported-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>