Merge branch 'ps/reftable-portability'

Update reftable library part with what is used in libgit2 to improve portability to different target codebases and platforms. * ps/reftable-portability: reftable/system: add abstraction to mmap files reftable/system: add abstraction to retrieve time in milliseconds reftable/fsck: use REFTABLE_UNUSED instead of UNUSED reftable/stack: provide fsync(3p) via system header reftable: introduce "reftable-system.h" header