Mask the file extension with .vcs in the annotate buffer

Without the .vcs extension, autocommand will try to run commands for the
filetype associated with the extension.  This has caused errors for me
when using vimerl as compiler/erlang.vim executes ShowErrorMsg each time
I hit a key in the annotate buffer

TBH I don't really know how ShowErrorMsg is executing without the
errorlist variable being set, so I won't pull this back into the main
repo