Add test for blob/tree name collisions in index

tests/index/collision.c

+#include "clar_libgit2.h"
+#include "git2/repository.h"
+#include "git2/index.h"
+
+git_repository *repo = NULL;
+
+void test_index_collision__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+	repo = NULL;
+}
+
+void test_index_collision__add(void)
+{
+	git_index *index;
+	git_index_entry entry;
+	git_oid tree_id;
+	git_tree *tree;
+
+	repo = cl_git_sandbox_init("empty_standard_repo");
+	cl_git_pass(git_repository_index(&index, repo));
+
+	memset(&entry, 0, sizeof(entry));
+	entry.ctime.seconds = 12346789;
+	entry.mtime.seconds = 12346789;
+	entry.mode  = 0100644;
+	entry.file_size = 0;
+	git_oid_fromstr(&entry.id, "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391");
+
+	entry.path = "a/b";
+	cl_git_pass(git_index_add(index, &entry));
+
+	/* create a tree/blob collision */
+	entry.path = "a/b/c";
+	cl_git_fail(git_index_add(index, &entry));
+
+	cl_git_pass(git_index_write_tree(&tree_id, index));
+	cl_git_pass(git_tree_lookup(&tree, repo, &tree_id));
+
+	git_tree_free(tree);
+	git_index_free(index);
+}