`magit-merge': update docstring

See commit 6fcac6c5.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-merge-check-clean': optimize

Check `magit-merge-warn-dirty-worktree' *before* `magit-everything-clean-p'.

Evaluating a variable is much cheaper than making two calls to `git-diff(1)',
so do that first instead of wasting cycles only to find out we weren't planning
on asking for confirmation anyway.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #930 from praet/merge-tweaks

Merge tweaks