Add a script for verifying Git-SVN mirrors

It was written as a one-off, intended to solve a problem one time only,
and not intended for continued use. But it may be useful at some point
in the future too, so here it is.
Add readme for one-offs folder
git-find: add brief description
svn-to-git: add discouraging note
update-macports: add note about Homebrew
vig: use superior $() syntax
Move a few scripts into the one-offs section
Move obsolete scripts into a separate section