enhance: do not refresh local changes for files written into ignored directories - #2873

…directories

Builds write continuously into ignored output folders (`bin/`, `obj/`,
`node_modules/`, `target/`...), and each of these writes scheduled a full
`git status`, although such changes can never appear in it.

The watcher now asks git (`status --ignored=matching`) which directories are
entirely ignored and skips the events under them. The list is queried lazily,
dropped when the index, a `.gitignore` or `info/exclude` changes (tracked files
or ignore rules may have changed), and queried again when a new directory is
created (the output folder of a first build, for example).

The result of `IsInSubmodule` is also cached per directory instead of probing
the file system up to the root for every event. The cache is cleared when
`.gitmodules` or a nested `.git` changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KCxH1zwDXLd9kfAAkcf6PZ