util: use tuple accessor to get accurate st_mtime value (issue4836)
Because st.st_mtime is computed as 'sec + 1e-9 * nsec' and double is too narrow
to represent nanoseconds, int(st.st_mtime) can be 'sec + 1'. Therefore, that
value could be different from the one got by osutils.listdir().
This patch fixes the problem by accessing to raw st_mtime by tuple index.
It catches TypeError to fall back to st.st_mtime because our osutil.stat does
not support tuple index. In dirstate.normal(), 'st' is always a Python stat,
but in dirstate.status(), it can be either a Python stat or an osutil.stat.
Thanks to vgatien-baron@janestreet.com for finding the root cause of this
subtle problem.