bin/meld: Don't pass git environment vars on to children (bgo#741142)

GIT_DIR and GIT_WORK_TREE are somewhat-helpfully set by some helper
tools such as git-gui. However, since we are quite good about setting
the cwd everywhere we call our git helpers, this sometimes messes with
things for us when invoking from subdirectories and whatnot. Since
these vars aren't helping us anyway, we'll just unset them in Meld's
environment altogether.