feature suggestion

Hi all,

Problem (scenario):

1. A file `file.xyz` has been added and commited
2. Some time after, let's say `*.xyz` is added to `.gitignore`
(probably by another contributor)
    Nobody notices that `file.xyz` is already tracked by git because
the file is not modified.
3. Another person accidentally changes `file.xyz` (may be it some
local config etc.), but no problem it is ignored anyway,... wait, why
does it shows up in `git status` as modified? Maybe it is not in
.gitignore? ... no it is... you get the point.

The suggestion is that when the .gitignore is updated, some kind of `git ls-files --others -i --exclude-standard` is rerun and if there are tracked files that are ignored due to the last update to .gitignore an appropriate warning is shown. This will eliminate the described surprise factor.

Sincerely,

-- Daniel Krikun