comm: fix comparison when reading from pipes

[martinkunkel2]

Use case is that two files are piped into comm, i.e. in bash

   comm <(cat file1) <(cat file2)

see also full example in #9537

Before the fix, comm reads from the pipes twice. Once in "fn comm" and once in "fn are_files_identical". As such, part of the data is skipped in comparison which leads to wrong output.

This is fixed by skipping the file comparison in case one of the files is not a regular file.

This fixes #9537

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GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

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GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)