magit-git-standard-options: add -c core.preloadindex=true
magit-get-current-tag: avoid using --dirty

Don't use the Git `--dirty' argument unless the function argument
WITH-DISTANCE-P is the symbol `dirty'.  This makes `git tag' much faster
on file-systems that don't properly cache access times.  Inside Magit we
currently are not ever interested in knowing whether there is any dirt,
but some third-party code does (well, some of my local hacks, that is).
Merge branch 'mushroom'