One of these may be the empty string, if either the autotools build or
meson build is missing.  Passing the variables bare will make the
command line think there's nothing there at all, but quoting an empty
string acts as an empty string command line argument, which 'git diff'
will get confused by.

Usually it's of course a good practice to quote variables that contain
filenames, since they could have spaces in them or other shell-unsafe
characters.  But in this case we know the variable will contain a string
that consists of only letters and dots.