fsck: detect gitmodules URLs with embedded newlines

fsck: detect gitmodules URLs with embedded newlines

The credential protocol can't handle values with newlines. We already
detect and block any such URLs from being used with credential helpers,
but let's also add an fsck check to detect and block gitmodules files
with such URLs. That will let us notice the problem earlier when
transfer.fsckObjects is turned on. And in particular it will prevent bad
objects from spreading, which may protect downstream users running older
versions of Git.

We'll file this under the existing gitmodulesUrl flag, which covers URLs
with option injection. There's really no need to distinguish the exact
flaw in the URL in this context. Likewise, I've expanded the description
of t7416 to cover all types of bogus URLs.