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