`magit-revert-item': don't depend on `cl-flet'

Commit 1c7fb237 replaced the `magit-with-revert-confirmation' macro with
the temporary function `confirm', but defined the latter using `cl-flet'
which isn't available in cl-lib 0.3, so use `let' + `funcall' instead.

Fixes #895.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-revert-item': respect `magit-revert-item-confirm'

Commit 1c7fb237 replaced the `magit-with-revert-confirmation' macro with
the temporary function `confirm', but in doing so also removed the check
for `magit-revert-item-confirm', causing `magit-revert-item' to always
ask for confirmation, even if the user configured it not to.

Fixes #896.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #897 from praet/revert

Revert