model: allow staging tracked files that are ignored

The cola interface only shows files that are either tracked or
untracked and not ignored.  Someone can have a file in their
.gitignore that matches a tracked file, and staging the file
will fail.  Use "git add --force" since we do not care about the
ignore rules when staging.

Signed-off-by: David Aguilar <davvid@gmail.com>