Unlike "$EDITOR" and "$GIT_EDITOR" that can hold the path to the
command and initial options (e.g. "/path/to/emacs -nw"), 'git p4'
did not let the shell interpolate the contents of the environment
variable that name the editor "$P4EDITOR" (and "$EDITOR", too).
Make it in line with the rest of Git, as well as with Perforce.
* ld/p4-editor-multi-words:
git-p4: tests: use test-chmtime in place of touch
git-p4: fix handling of multi-word P4EDITOR
git-p4: add failing test for P4EDITOR handling