Add add_gitignore mthod to WalkBuilder

[shulaoda]

Describe your feature request

Related to #2964

Currently, OverrideBuilder does not behave as expected with respect to the reverse functionality of GitignoreBuilder. Therefore, I propose adding an add_gitignore method to WalkBuilder that would allow users to directly pass a Gitignore object instead of needing to provide a path that converts a .gitignore file into a Gitignore object.

This method could function similarly to add_ignore, but instead of reading the .gitignore file from a path, it would accept a Gitignore object directly.

pub fn add_gitignore(&mut self, gi: Gitignore)  {
    self.ig_builder.add_ignore(gi);
}