Merge branch 'ds/advice-sparse-index-expansion'

A new warning message is issued when a command has to expand a
sparse index to handle working tree cruft that are outside of the
sparse checkout.

* ds/advice-sparse-index-expansion:
  advice: warn when sparse index expands

diff --cc Documentation/config/advice.txt
Simple merge

diff --cc advice.c
Simple merge

diff --cc advice.h
Simple merge

diff --cc sparse-index.c
Simple merge