builtin/repack.c: use named flags for existing_packs

builtin/repack.c: use named flags for existing_packs

We use the `util` pointer for items in the `existing_packs` string list
to indicate which packs are going to be deleted. Since that has so far
been the only use of that `util` pointer, we just set it to 0 or 1.

But we're going to add an additional state to this field in the next
patch, so prepare for that by adding a #define for the first bit so we
can more expressively inspect the flags state.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>