Git plugin should allow configurable checkout location

The Git and Mercurial plugins (see #3602) behave differently from the SVN plugin: the latter checks the repo out as a subdirectory of the workspace while the former two check the repo out as the workspace. This is problematic in environments where multiple VCSes are in use but projects would like to share build tools as the locations will either be $WORKSPACE/repo_name or $WORKSPACE depending on the VCS in use; it's also often nice to have other top-level directories which aren't in the local repo.

This could be solved simply by having the other plugins offer the ability to check out into a subdirectory.