cat: error when output is input and appending

[jfinkels]

Change cat so that it terminates with an error message when the input file is the same as the output file and the output file is being appended to. For example,

cat <f >>f
cat: -: input file is output file

Fixes #7165

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)

[sylvestre]

Is it possible to add a test for this?

[jfinkels]

There probably is a way to test it, but I'm not sure how to do it from inside Rust. We just need to do something like

touch f
cat <f >>f

and check for the error.

[RenjiSann]

Here is a test that checks the behavior.

It's the occasion to shout out to the people who engineered the testsuite. It's really easy to use, and it works like a charm. Thank you very much, testsuite maker 🙏

[jfinkels]

Thanks @RenjiSann, I've included your commit in this branch.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[RenjiSann]

Actually, I have not checked that it worked correctly on every platform, and I haven't cleaned it either, that's why CI is failing ^^;

[github-actions]

GNU testsuite comparison:

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

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)

[RenjiSann]

The changes look good to me 👌 :

Please squash all the commits related to the test in a single one to reduce verbosity on the main branch, and e'll be good to go 👍

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[RenjiSann]

Thank you for the change !