Overwrite ~/.rbenv/.git/config after cloning repos

[sinsoku]

Probably the cause is as follows.

1. rev-parse --is-inside-work-tree return true in subdirectoires of rbenv.
   • https://github.com/amatsuda/gem-src/blob/v0.9.0/lib/rubygems_plugin.rb#L45
2. remote_add_src_and_origin add remotes to ~/.rbenv/.git/config when gem_dir not a repository.
   • https://github.com/amatsuda/gem-src/blob/v0.9.0/lib/rubygems_plugin.rb#L50