Merge branch 'dt/unpack-save-untracked-cache-extension'

When "git checkout", "git merge", etc. manipulates the in-core
index, various pieces of information in the index extensions are
discarded from the original state, as it is usually not the case
that they are kept up-to-date and in-sync with the operation on the
main index.  The untracked cache extension is copied across these
operations now, which would speed up "git status" (as long as the
cache is properly invalidated).

* dt/unpack-save-untracked-cache-extension:
unpack-trees: preserve index extensions

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc unpack-trees.c
Simple merge