This makes the workging copy walk skip an entire ignored directory if
there are no negative patterns later in the ignore file. That speeds
up `jj st` in this repo with ~13k files in `target/` from ~100 ms to
~25 ms (6.0dB). This closes issue #8.
This is to address issue #8. I haven't added the optimization to avoid
walking all the files in `target/` yet. Even so, this patch still
speeds up `jj st` in this repo, with ~13k files in `target/`, from
~320 ms to ~100 ms (-5.1dB). The time actually checking if paths match
gitignores seems to go down from 116 ms to 6 ms. I think that's mostly
because libgit2 has to look for `.gitignore` files in every parent
directory every time we ask it about a file, while the rewritten code
looks for a `.gitignore` file only when visiting a new directory.
