Changed invocation of editor to something that might be more
portable (though perhaps only on Linux). Also changed logic for finding
top-level of git-repository to something better and less convoluted. The
old way I used of finding it had some odd behaviour; when I was in a
directory beneath the top-level, the old way worked fine because one of
the commands returned an absolute path. But when I was in the top-level
and invoked the script, then one of the commands would return
the *relative* path to the top-level, which made everything screwy since
I assumed and relied on an absolute path.