gitignores: add own implementation and stop using libgit2's

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.

Author: martinvonz

Cargo.lock

@@ -30,6 +30,7 @@ pest = "2.1.3"
 pest_derive = "2.1.0"
 protobuf = { version = "2.22.1", features = ["with-bytes"] }
 rand = "0.8.0"
+regex = "1.5.4"
 serde_json = "1.0.60"
 tempfile = "3.1.0"
 thiserror = "1.0.22"