Default files for projectile-grep with prefix arg

When prefix arg of '- is given (such as M--):
 If we have a `buffer-file-name' default (without prompt) the files
 to what `grep-read-files' would return.
When prefix arg is something other than '-, then prompt as before, but
 with a default of what `grep-read-files' would return if we have a
 `buffer-file-name'.

This is useful for multi-language projects in particular.