The struct declaration and implementation macros are moved to
commit-slab-hdr.h and commit-slab-impl.h respectively.
This right now is not needed for current users but if we make a public
commit-slab type, we may want to avoid including the slab
implementation in a header file which gets replicated in every c file
that includes it.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
