cp: fix --remove-destination deleting source when dest is the same file under a different path

[LoukasPap]

Fixes #13103

Problem

It compared the source and destination inputs as passed, so for example "a" and "./a" were compared and were -of course- found different.

Solution

We convert them to absolute paths using canonicalize(...) and then perform the equality check.

[codspeed-hq]

Merging this PR will not alter performance

✅ 323 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing LoukasPap:issue13103 (070a2de) with main (e73b8a6)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout-group (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[LoukasPap]

Is there a way to test the performance locally or before pushing?

[LoukasPap]

Changed order of conditions to allow same-file check to happen only when --remove-destination param is set, thus avoiding redundant comparison of source and dest.