fix timestamps (but stays on Irmin 0.10.1)

[hannesm]

is on top of #43, cleaner code which also works with 0.10.1 (maybe too much work with 0.11.0 where the Topographical.fold seems to be sorted by date)

[hannesm]

doesn't seem to be good, on a git push to the repo, I get from canopy:

Raised at file "bytes.ml", line 220, characters 25-34
Called from file "src/pre_sexp.ml", line 86, characters 11-45
149.158 mirage       ERROR: Git_mirage.IO.read_exactly: expecting 4, got 0
Fail pull (Failure "Git_mirage.IO.read_exactly: expecting 4, got 0")

[Engil]

It happens only on this revision ?
Because I have somewhere a nasty ocaml-git bug that happens on some specific repos, and this might be this one.

[hannesm]

not sure... also the first clone works fine, but subsequent pulls seem to not work properly...