ui-plain: more efficient walk_tree()

Git's read_tree_recursive() already filters out the objects by pathname,
so we only have to compare baselen to the expected.  That is, no string
matching is required.

Additionally, if the requested path is a directory, the old code would
walk through all of its immediate children.  This is not necessary, this
so we no longer do that.

Signed-off-by: Mark Lodato <lodatom@gmail.com>