range-diff: plug memory leak in read_patches()

range-diff: plug memory leak in read_patches()

Amend code added in d9c66f0b5bf (range-diff: first rudimentary
implementation, 2018-08-13) to use a "goto cleanup" pattern. This
makes for less code, and frees memory that we'd previously leak.

The reason for changing free(util) to FREE_AND_NULL(util) is because
at the end of the function we append the contents of "util" to a
"struct string_list" if it's non-NULL.

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