Not the answer to your specific question, but I find the best solution. Why not back up all your files to a separate directory, regardless of whether they are in the original control?
Here's how, from my .vimrc. This creates backups in the ~ / .vim_backups shared directory:
"Use a common directory for backups and swp files
"Create it if it doesn't exist
silent execute '! mkdir -p ~ / .vim_backups'
set backupdir = ~ / .vim_backups //
set directory = ~ / .vim_backups //
hgmnz source share