Split url by whitespace: '.git' might not exist

Ref: https://github.com/jaxbot/github-issues.vim/issues/7
Use `new` instead of `split`; use repo in buffer name
Use `buffer[:]` to replace the whole buffer with error
Trim trailing whitespace
map: use `normal!` to skip custom maps; `<C-w>p` is unnecessary
Merge pull request #10 from blueyed/minor-fixes

Minor fixes