repo create: withhold the hint a doubled suffix would poison

"widgets.git.git" was answered with (use "widgets.git"), and that name is refused by the very guard that offered it -- the user is told a rule, follows it, and is told the same rule again.

The shape checks could not catch this, because nothing about the remainder is malformed. The cut runs exactly once, so "widgets.git.git" leaves "widgets.git": an interior dot, which nativeRepoRe rightly allows. The only thing wrong with it is the reserved suffix, so that is what has to be asked.

The fixture-driven test missed it too, and the reason is worth keeping. It asserted the error merely CONTAINED the remainder, which passes trivially: the message already quotes the name the user typed, and "widgets.git.git" contains "widgets.git". It now asserts the whole parenthetical, and asserts its absence for every remainder that still carries the suffix.

Reported by Copilot on #2630.

Refs COR-1892