Added the scripts git-svnup/git-svnrevert to utils/git-svn.

It makes more sense to have git-svnup here than catting said file in the
documentation (where we should rather point users to this directory).
I included git-svnrevert as an additional gift to the community. I will update
the documentation in a second commit later today.

git-svnrevert takes in a git hash for a commit, looks up the svn revision for
said commit and then creates the normal git revert commit message with the one
liner message, except instead of saying

  Revert "<<<INSERT ONELINER HERE>>>"

  This reverts commit <<<INSERT GITHASH HERE>>>

It says:

  Revert "<<<INSERT ONELINER HERE>>>"

  This reverts commit r<<<INSERT SVN REVISION HERE>>>

so git hashes will not escape into our svn logs (which just look unseemly).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180587 91177308-0d34-0410-b5e6-96231b3b80d8