Make index_insert keep existing case

In a case insensitive index, if you attempt to add a file from
disk with a different case pattern, the old case pattern in the
index should be preserved.

This fixes that (and a couple of minor warnings).

src/index.c

@@ -734,8 +734,9 @@ static int index_insert(git_index *index, git_index_entry *entry, int replace)
 	if (!replace || !existing)
 		return git_vector_insert(&index->entries, entry);
 
-	/* exists, replace it */
-	git__free((*existing)->path);
+	/* exists, replace it (preserving name from existing entry) */
+	git__free(entry->path);
+	entry->path = (*existing)->path;
 	git__free(*existing);
 	*existing = entry;
 

tests-clar/diff/rename.c

@@ -911,7 +911,6 @@ void test_diff_rename__case_changes_are_split(void)
 {
 	git_index *index;
 	git_tree *tree;
-	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
 	git_diff_list *diff = NULL;
 	diff_expects exp;