Merge branch 'ms/update-index-racy'

"git update-index --refresh" has been taught to deal better with
racy timestamps (just like "git status" already does).

* ms/update-index-racy:
  update-index: refresh should rewrite index in case of racy timestamps
  t7508: add tests capturing racy timestamp handling
  t7508: fix bogus mtime verification
  test-lib: introduce API for verifying file mtime

diff --cc cache.h
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge