mv: replace src_dir with a strvec
We manually manage the src_dir array with ALLOC_GROW. Using a strvec is
a little more ergonomic, and makes the memory ownership more clear. It
does mean that we copy the strings (which were otherwise just pointers
into the "sources" strvec), but using the same rationale as
9fcd9e4e72
(builtin/mv duplicate string list memory, 2024-05-27), it's just not
enough to be worth worrying about here.
As a bonus, this gets rid of some "int"s used for allocation management
(though in practice these were limited to command-line sizes and thus
not overflowable).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>