diff: Don't compare a file or directory to itself

While here, stop abusing struct dirent for something we don't even need
to store.

PR: 254455
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj, kevans
Differential Revision: https://reviews.freebsd.org/D55113

(cherry picked from commit 590126789c841d80655869bc075c8980c173dd1c)

diff: Fix build

rc must be defined first.

Fixes: 590126789c84
MFC after: 1 week
X-MFC with: 590126789c84

(cherry picked from commit ee44ab936e84bacaa49847d36aabdf280f9fecce)