builtin/repack.c: rename various pack_geometry functions

builtin/repack.c: rename various pack_geometry functions

Rename functions which work with 'struct pack_geometry' to begin with
"pack_geometry_". While we're at it, change `free_pack_geometry()` to
instead be named `pack_geometry_release()` to match our conventions, and
make clear that that function frees the contents of the struct, not the
memory allocated to hold the struct itself.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>