Merge branch 'jk/commit-info-slab'

Allow adding custom information to commit objects in order to
represent unbound number of flag bits etc.

* jk/commit-info-slab:
commit-slab: introduce a macro to define a slab for new type
commit-slab: avoid large realloc
commit: allow associating auxiliary info on-demand

diff --cc commit.c
Simple merge

diff --cc commit.h
Simple merge