submodule-config: avoid memory leak

submodule-config: avoid memory leak

In 961b130d20c9 (branch: add --recurse-submodules option for branch
creation, 2022-01-28), a funny pattern was introduced where first some
struct is `xmalloc()`ed, then we resize an array whose element type is
the same struct, and then the first struct's contents are copied into
the last element of that array.

Crucially, the `xmalloc()`ed memory never gets released.

Let's avoid that memory leak and that memory allocation dance altogether
by first reallocating the array, then using a pointer to the last array
element to go forward.

Reported by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>