Fix handling of absolute patterns in parent gitignore files.

If a gitignore file in a *parent* directory is used, then it must be
matched relative to the directory it's in. ripgrep wasn't actually
adhering to this rule. Consider an example:

  .gitignore
  src
    llvm
      foo

Where `.gitignore` contains `/llvm/` and `foo` contains `test`. When
running `rg test` at the top-level directory, `foo` is correctly searched.
If you `cd` into `src` and re-run the same search, `foo` is ignored because
the `/llvm/` pattern is interpreted with respect to the current working
directory, which is wrong. The problem is that the path of `llvm` is
`./llvm`, which makes it look like it should match.

We fix this by rebuilding the directory path of each file when traversing
gitignores in parent directories. This does come with a small performance
hit.

Fixes #25.

tests/tests.rs

@@ -609,6 +609,22 @@ clean!(regression_16, "xyz", ".", |wd: WorkDir, mut cmd: Command| {
     wd.assert_err(&mut cmd);
 });
 
+// See: https://github.com/BurntSushi/ripgrep/issues/25
+clean!(regression_25, "test", ".", |wd: WorkDir, mut cmd: Command| {
+    wd.create(".gitignore", "/llvm/");
+    wd.create_dir("src/llvm");
+    wd.create("src/llvm/foo", "test");
+
+    let lines: String = wd.stdout(&mut cmd);
+    let expected = "src/llvm/foo:test\n";
+    assert_eq!(lines, expected);
+
+    cmd.current_dir(wd.path().join("src"));
+    let lines: String = wd.stdout(&mut cmd);
+    let expected = "llvm/foo:test\n";
+    assert_eq!(lines, expected);
+});
+
 // See: https://github.com/BurntSushi/ripgrep/issues/49
 clean!(regression_49, "xyz", ".", |wd: WorkDir, mut cmd: Command| {
     wd.create(".gitignore", "foo/bar");