vcsdiff: handle modified buffer

[blueyed]

It seems to be nice having VCS signs updated even when a file is not saved, i.e. for a modified file.

I am not sure if this should be done by default, but https://github.com/airblade/vim-gitgutter does it. It could be looked into for supporting this with Git, but it might even be not possible for other VCSs supported by quickfixsigns.

What do you think?