Pkg.tag: add force=true keyword to force retagging [fix #4617].
Pkg.publish: refuse if a version SHA1 has been changed [#4617].

This is the protection from the power introduced by the previous
commit: it allows you to force write tags, but this prevents you
from publishing changes to existing published version SHA1s.
Merge branch 'master' of github.com:JuliaLang/julia