fix(tvix/nar-bridge): Fix directory stack tracking

Previously, nar-bridge, had a couple of bugs with tracking the current
directory when traversing a NAR file.

The included test case looks like:
```
/ (dir)
/test (dir)
/test/tested (file)
/tested (file)
```

Previously, we would do a string prefix match between the current node
and the top of the directory stack to determine if the node is in the
directory. In this case `/test` is a substring of `/tested`; however,
`/tested` is not in the `/test` directory. The fix is to append a `/` to
the directory name when doing the prefix match, so `/test/` is not a
prefix of `/tested`.

Additionally, when popping the stack, we need to continuously pop the
stack until the new node is in the directory at the top of the stack
(stopping before we pop the root directory)

Example:
```
/ (dir)
/a (dir)
/a/b (dir)
/a/b/c (file)
/z (file)
```

Previously, `z` would end up in directory `/a` because we only the pop
the stack once.

The included test case requires both of these issues to be fixed for it
to pass, so I think it is sufficient.

Change-Id: I22f601babf04d39d85535ba7ad585d3970757211
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9348
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: Connor Brewster <cbrewster@hey.com>

tvix/nar-bridge/pkg/reader/reader_test.go

@@ -566,3 +566,31 @@ func TestCallbackErrors(t *testing.T) {
 		require.ErrorIs(t, err, targetErr)
 	})
 }
+
+// TestPopDirectories is a regression test that ensures we handle the directory
+// stack properly.
+//
+// This test case looks like:
+//
+// / (dir)
+// /test (dir)
+// /test/tested (file)
+// /tested (file)
+//
+// We used to have a bug where the second `tested` file would appear as if
+// it was in the `/test` dir because it has that dir as a string prefix.
+func TestPopDirectories(t *testing.T) {
+	f, err := os.Open("../../testdata/popdirectories.nar")
+	require.NoError(t, err)
+	defer f.Close()
+
+	r := reader.New(f)
+	_, err = r.Import(
+		context.Background(),
+		func(fileReader io.Reader) error { return nil },
+		func(directory *storev1pb.Directory) error {
+			return directory.Validate()
+		},
+	)
+	require.NoError(t, err)
+}