head: fix meaning of ---presume-input-pipe

[bvinc]

The option ---presume-input-pipe is debug option that should disable the seeking optimization, not force it to read stdin. This fixes the GNU test head-elide-tail.

[bvinc]

The GNU test head/head-elide-tail was failing because it was running head on a file with the option ---presume-input-pipe. This misinterpretation of the meaning of the option was causing head to erroneously read from stdin, which was causing the process to be suspended with signal SIGTTIN which is what happens when a background process tries to read from stdin.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/usage_vs_getopt (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/head/head-elide-tail is no longer failing!

[cakebaker]

Can you please add a test so that we don't regress in the future? Thanks!

[bvinc]

It's interesting to add a test for an argument that is not supposed to change behavior at all. But I've added a simple one that would have failed previously.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/head/head-elide-tail is no longer failing!

[cakebaker]

Congrats! The gnu test tests/head/head-elide-tail is no longer failing!

Great, thanks!