Use 'git rev-parse --show-prefix' to test if at top level.
Use 'git rev-parse --show-prefix' to test if at top level.

This check lets git-subrepo work within submodules.

ingydotnet/git-subrepo,perlpunk/git-subrepo,perlpunk/git-subrepo,ingydotnet/git-subrepo,stsquared99/git-subrepo,stsquared99/git-subrepo