nice: untangle complex match

[cakebaker]

This PR untangles a complex match by moving some logic to a let else so that we can match on a single value instead of two. It also removes a mut by using two variables instead of one.

[oech3]

Do we still avoid parsing twice?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[cakebaker]

@oech3 which parsing do you mean? The parsing of the value of --adjustment?

[oech3]

and cmd_iter too.