`magit-apply-hunk-item*' call `use-region-p' only once

Commit 83dac351 replaced `magit-use-region-p' with `use-region-p',
but in doing so, caused us to call `use-region-p' twice.

Also, the function might at some point in the future be changed to do
stuff which alters or unsets the region between the first and second
check, which would result in a bug:  Defensive Programming FTW.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #834 from praet/use-region

`magit-apply-hunk-item*' call `use-region-p' only once