ls-files: conditionally leave index sparse

When running 'git ls-files' with a pathspec, the index entries get filtered according to that pathspec before iterating over them in show_files(). In 78087097b8 (ls-files: add --sparse option, 2021-12-22), this iteration was prefixed with a check for the '--sparse' option which allows the command to output directory entries; this created a pre-loop call to ensure_full_index(). However, when a user runs 'git ls-files' where the pathspec matches directories that are recursively matched in the sparse-checkout, there are not any sparse directories that match the pathspec so they would not be written to the output. The expansion in this case is just a performance drop for no behavior difference. Replace this global check to expand the index with a check inside the loop for a matched sparse directory. If we see one, then expand the index and continue from the current location. This is safe since the previous entries in the index did not have any sparse directories and thus would remain stable in this expansion. A test in t1092 confirms that this changes the behavior. Signed-off-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>