Merge branch 'sy/sparse-grep'

"git grep" learned to expand the sparse-index more lazily and on
demand in a sparse checkout.

* sy/sparse-grep:
  builtin/grep.c: integrate with sparse index

t/perf/p2000-sparse-operations.sh

@@ -124,5 +124,6 @@ test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
 test_perf_on_all "git rm -f $SPARSE_CONE/a && git checkout HEAD -- $SPARSE_CONE/a"
+test_perf_on_all git grep --cached --sparse bogus -- "f2/f1/f1/*"
 
 test_done