Prevent creating `..`, `.`, and `.git` with tree builder

As per core git.

src/tree.c

@@ -26,7 +26,9 @@ static bool valid_filemode(const int filemode)
 
 static int valid_entry_name(const char *filename)
 {
-	return *filename != '\0' && strchr(filename, '/') == NULL;
+	return *filename != '\0' && strchr(filename, '/') == NULL &&
+		strcmp(filename, "..") != 0 && strcmp(filename, ".") != 0 &&
+		strcmp(filename, ".git") != 0;
 }
 
 static int entry_sort_cmp(const void *a, const void *b)

tests-clar/object/tree/write.c

@@ -39,6 +39,12 @@ void test_object_tree_write__from_memory(void)
 		&bid, GIT_FILEMODE_BLOB));
 	cl_git_fail(git_treebuilder_insert(NULL, builder, "/",
 		&bid, GIT_FILEMODE_BLOB));
+	cl_git_fail(git_treebuilder_insert(NULL, builder, ".git",
+		&bid, GIT_FILEMODE_BLOB));
+	cl_git_fail(git_treebuilder_insert(NULL, builder, "..",
+		&bid, GIT_FILEMODE_BLOB));
+	cl_git_fail(git_treebuilder_insert(NULL, builder, ".",
+		&bid, GIT_FILEMODE_BLOB));
 	cl_git_fail(git_treebuilder_insert(NULL, builder, "folder/new.txt",
 		&bid, GIT_FILEMODE_BLOB));