add: skip tracked paths outside sparse-checkout cone

When 'git add' adds a tracked file that is outside of the
sparse-checkout cone, it checks the SKIP_WORKTREE bit to see if the file
exists outside of the sparse-checkout cone. This is usually correct,
except in the case of a merge conflict outside of the cone.

Modify add_pathspec_matched_against_index() to be more careful about
paths by checking the sparse-checkout patterns in addition to the
SKIP_WORKTREE bit. This causes 'git add' to no longer allow files
outside of the cone that removed the SKIP_WORKTREE bit due to a merge
conflict.

With only this change, users will only be able to add the file after
adding the file to the sparse-checkout cone. A later change will allow
users to force adding even though the file is outside of the
sparse-checkout cone.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/pathspec.c b/pathspec.c
index 44306fd..ddeeba7 100644
--- a/pathspec.c
+++ b/pathspec.c
@@ -39,7 +39,8 @@ void add_pathspec_matches_against_index(const struct pathspec *pathspec,
 		return;
 	for (i = 0; i < istate->cache_nr; i++) {
 		const struct cache_entry *ce = istate->cache[i];
-		if (sw_action == PS_IGNORE_SKIP_WORKTREE && ce_skip_worktree(ce))
+		if (sw_action == PS_IGNORE_SKIP_WORKTREE &&
+		    (ce_skip_worktree(ce) || !path_in_sparse_checkout(ce->name, istate)))
 			continue;
 		ce_path_match(istate, ce, pathspec, seen);
 	}
@@ -70,7 +71,7 @@ char *find_pathspecs_matching_skip_worktree(const struct pathspec *pathspec)
 
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce = istate->cache[i];
-		if (ce_skip_worktree(ce))
+		if (ce_skip_worktree(ce) || !path_in_sparse_checkout(ce->name, istate))
 		    ce_path_match(istate, ce, pathspec, seen);
 	}