bin/mdw-build: Initialize `gitver' even when not checking out.

bin/mdw-build: Initialize `gitver' even when not checking out.

Otherwise we still try to extend the changelog file, only `gitver' is
empty, so the `new' version isn't actually different from the previous
one and `lintian' gets sad.

This setting includes the standard `+' for a dirty tree, which is
clearly superfluous in a `checkout' build.