oidset: introduce 'oidset_size'

oidset: introduce 'oidset_size'

Occasionally, it may be useful for callers to know the number of object
IDs in an oidset. Right now, the only way to compute this is to call
'kh_size' on the internal 'kh_set_oid_t'.

Similar to how we wrap other 'kh_*' functions over the 'oidset' type,
let's allow callers to compute this value by introducing 'oidset_size'.

We will add its first caller in the subsequent commit.

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