dmd v2.076.0-dirty?

[Gary Willoughby via Digitalmars-d]

I've just installed the latest dmd version (on ubuntu 64bit) and 
I get this:

$ dmd --version
DMD64 D Compiler v2.076.0-dirty
Copyright (c) 1999-2017 by Digital Mars written by Walter Bright

What does 'dirty' mean?