Mark excluded files in toggle_git_file_info

[kidonng]

It would be awesome excluded files are given special marks when toggle_git_file_info is enabled, in addition to current marks (e.g. M for modified).

Example: eza (an alternative ls implementation) supports listing files with git file info. Notice how in the screenshot some files are marked with an I, which means the files are ignored by Git.

[Canop]

The reason I didn't care about the Ignored status is that you can only see those marks when ignored files are shown so I found it to be useless to me. But I have no objection against having it.

[Canop]

@kidonng can you confirm that #917 seems to do what you want ?