seq: improve format_float_shortest to match GNU

[sylvestre]

Should help with tests/cp/link-heap

Closes: #5766

[github-actions[bot]]

GNU testsuite comparison:

GNU test failed: tests/seq/seq-extra-number. tests/seq/seq-extra-number is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is no longer ERROR!

[tertsdiepraam]

Closing since #5766 was closed