oidset: uninline oidset_init()

oidset: uninline oidset_init()

There is no need to inline oidset_init(), as it's typically only called
twice in the lifetime of an oidset (once at the beginning and at the end
by oidset_clear()) and kh_resize_* is quite big, so move its definition
to oidset.c.  Document it while we're at it.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>