Work around 'wildignore' conflicts.

Use s:fnameescape instead of backtick expressions.

References #121.

I removed `.git` from `wildignore` long ago, but IT recently changed my environment and I'm getting errors related to this again, presumably because of some other thing in `wildignore`. 

My perspective: If wildignore is of any use, one shouldn't have to worry that it's going to break plugins. I use `wildignore` for `:vimgrep` and `find()`, but this user-facing option should not affect the internal operation of plugins, for the same reason that plugins call `:norm!` instead of `:norm`.