submodule--helper: free some "displaypath" in "struct update_data"

submodule--helper: free some "displaypath" in "struct update_data"

Make the update_data_release() function free "displaypath" member when
appropriate. The "displaypath" member is always ours, the "const" on
the "char *" was wrong to begin with.

This leaves a leak of "displaypath" in update_submodule(), which as
we'll see in subsequent commits is harder to deal with than this
trivial fix.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>