make dfm not assume that it's in the bin subdirectory of the dotfiles repo

[justone]

Right now, if dfm is running from inside a dotfiles repo, which is the normal case, it assumes that it's in the bin subdirectory of that repo, because that's how the original dotfiles repo is set up.

dfm should support being anywhere in the dotfiles repo, if it's in a dotfiles repo at all.

[justone]

Reference: getting the git root dir in one command