Delete the name-duplicating buffer before ceating a gist file buffer

[iiifr]

If repeatly opening a same file on the vim-gist, vim-gist gives the new buffer the same name as previous buffer, without deleting the opened buffer first. Then a file E95 exception is thrown, and vim-gist will echo error message Gist contains binary.

Probably corressponding issue report: https://github.com/mattn/vim-gist/issues/220

[xbot]

@mattn Hi, mind merging this PR ASAP? I just came across this problem too.