fix install same file with backup flag

[zhw2101024]

Fixes #12696.
In fact, whenever we use the backup flag with the same file, it need to report that the src and dst are the same file.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[zhw2101024]

@sylvestre all tests have passed, please have a look.