Support for git-svn

Neither git or svn plugin can be used with release plugin on a svn repository checked out using git-svn, so we need probably another maven-scm-provider-git-svn or support for that in one of them.

It will probably need to execute a mix of the two, using "git svn tag" for tagging after "git svn dcommit" etc.

As it is used against a svn repository, it would be best if no configuration needs to be changed. Thus either add a command line parameter to enable git-svn mode, or better, in plugin for svn, detect that the checkout is done using git-svn (the directory .git?), and use that module instead.