Merge branch 'ds/ls-files-lazy-unsparse'

Merge branch 'ds/ls-files-lazy-unsparse'

"git ls-files <pathspec>..." should not necessarily have to expand
the index fully if a sparsified directory is excluded by the
pathspec; the code is taught to expand the index on demand to avoid
this.

* ds/ls-files-lazy-unsparse:
  ls-files: conditionally leave index sparse