add: use unsigned type for collection of bits

add: use unsigned type for collection of bits

The 'refresh' function in 'builtin/add.c' declares 'flags' as
signed, and passes it as an argument to the 'refresh_index'
function, which though expects an unsigned value.

Since in this case 'flags' represents a bag of bits, whose MSB is
not used in special ways, change the type of 'flags' to unsigned.

Signed-off-by: Eugenio Gigante <redacted>
Signed-off-by: Junio C Hamano <redacted>