Only add `global` to gemsets when specified is not `global`.

* If I specified `global` in .rbenv-gemsets is the gemset
  what I want to use, then `rbenv gemset active` will output
  `global global`. We should consider this situation.
Display global when specify gemset.
Revert "Display global when specify gemset."

This reverts commit 3c4d34881be72ffcdafd0ba56a90d43bf55e2590.
Merge branch 'larrylv-master'