magit-ediff: correct doc-string

The range is passed directly to `git diff' which, when given
a single commit, compares the working tree with that commit.
Merge branch 'master' into next

Conflicts:
	magit.el
don't call magit-refresh-function if nil

The check against magit-refresh-function was dropped in
8aa75bf025f9353642c719a46b0257d1467ee90d, when magit-mode-refresh-buffer
was changed to magit-refresh-buffer.
Merge pull request #1434 from npostavs/safe-refresh

don't call magit-refresh-function if nil
magit-completing-read-function: advertise a helm variant