submodule: sparse-index should not collapse links

A submodule is stored as a "Git link" that actually points to a commit
within a submodule. Submodules are populated or not depending on
submodule configuration, not sparse-checkout. To ensure that the
sparse-index feature integrates correctly with submodules, we should not
collapse a directory if there is a Git link within its range.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sparse-index.c

@@ -52,6 +52,7 @@ static int convert_to_sparse_rec(struct index_state *istate,
 		struct cache_entry *ce = istate->cache[i];
 
 		if (ce_stage(ce) ||
+		    S_ISGITLINK(ce->ce_mode) ||
 		    !(ce->ce_flags & CE_SKIP_WORKTREE))
 			can_convert = 0;
 	}