The `issue comment` command would perform a `filepath.Base()` on the
value of EDITOR and use the result for launching the editor, which is
undesireable for both functional and display purposes:
- For functional purposes, always shellsplit the value of GH_EDITOR or
EDITOR before we shell out to that process;
- For display purposes, extract the basename only after shellsplitting.
If we are allowed to skip the editor _and_ we want to append the default
text to the editor if we'd opened it, we just return the default text.
Co-Authored-By: Mislav Marohnić <mislav@github.com>
