Improve support of ignored files for file history

[fcollonval]

In the file browser context menu, it is possible to request the file Git history through the submenu Git -> History. Unfortunately that entry is displayed for all files including those ignored by Git. It will be nice to know if a file is ignored to avoid displaying that entry for them.

Originally posted by @navn-r in #980 (comment)

[fcollonval]

To tackle this one, the best is to make use of git check-ignore command as it supports to check for all elements within a directory; e.g.

❯ git check-ignore ./*
./build
./jupyterlab_git.egg-info
./lib
./node_modules
./__pycache__
./tsconfig.tsbuildinf

To achieve this, we need to implement:

• in the backend:
  • in git.py#Git, add a new method check_ignore that execute git check-ignore -z <path> and parse the result to return the list of ignored files; changed_files method is a good example of a similar method.
  • in handlers.py#GitIgnoreHandler, add a get method that calls the new Git.check_ignore.
• in the frontend:
  • in model.ts#GitExtension, add a isIgnored(path) method that will request the new GET endpoint.
  • in commandsAndMenu.tsx#addFileBrowserContextMenu,
    • we need to remove from the items = toArray(filebrowser.selectedItems()) the ignored files; I let you look at the code to figure out how to achieve that.

cc @sheezaaziz