Make dir_tree() work even when glob or regexp is used

This PR addresses the issue described in r-lib#398. `dir_tree()` didn't work when a pattern was supplied to `regexp=` or `glob=` arguments that get passed on to `dir_ls()`. That's because the list of file paths returned by those calls to `dir_ls()` did not include paths of the parent (and grandparent etc.) directories that `dir_tree()` uses to construct the tree.

The code I've added uses the file paths returned by `dir_ls()` to recreate the directory paths that'll be needed to build the tree.

To see that this now works, try for example:

```r
dir_tree(system.file(package = "fs"), regexp = "[.]rds$")
dir_tree(system.file(package = "fs"), type = "directory")
```

R/tree.R

@@ -6,7 +6,16 @@
 #'
 #' @export
 dir_tree <- function(path = ".", recurse = TRUE, ...) {
+  find_ancestor_dirs <- function(path) {
+    parts <- path_split(path)[[1]]
+    Reduce(function(x, y) path_join(list(c(x, y))),
+           parts, accumulate = TRUE)
+  }
+
   files <- dir_ls(path, recurse = recurse, ...)
+  dd <- unlist(sapply(path_dir(files), find_ancestor_dirs))
+  dd <- setdiff(dd, path)
+  files <- unique(c(dd, files))
   by_dir <- split(files, path_dir(files))
 
   ch <- box_chars()