Merge branch 'ym/fix-opportunistic-index-update-race' into maint

"git status", even though it is a read-only operation, tries to update the index with refreshed lstat(2) info to optimize future accesses to the working tree opportunistically, but this could race with a "read-write" operation that modify the index while it is running. Detect such a race and avoid overwriting the index. * ym/fix-opportunistic-index-update-race: read-cache.c: verify index file before we opportunistically update it wrapper.c: add xpread() similar to xread()