Avoid cp -v in fixtures for portability

This change removes the `-v` option from the one `cp` invocation in
`test/fixtures/many.sh`, the one fixture script that uses it. This
is roughly analogous to the preceding removal of `-v` from the one
`mv` invocation that used that. But while `rm -v` is standardized
but only recently, `cp -v` is not standard. Even POSIX 1003.1-2024
does not require `cp` to recognize `-v`:

- https://pubs.opengroup.org/onlinepubs/9799919799/utilities/cp.html

See the preceding commit for general considerations, advantages and
disadvantages, and alterntive approaches. As there, this currently
just drops the `-v` without replacing it with anything, based on
the idea that printing the list of files being operated on (here,
copied) may not be important.

- That assumption may be less well-founded here, since the affected
  command, introduced in 482d6f3 (GitoxideLabs#1285), does *not* have other
  nearby `cp` commands that are conceptually related but omit `-v`.

- But, unlike there, it may be reasonable to consider the `-v` here
  to be a (minor) bug, since it is not standardized but it appears
  in test fixture code that seems intended to run all targets.

Author: EliahKagan

gix-dir/tests/fixtures/many.sh

@@ -280,7 +280,7 @@ git init expendable-and-precious-nested-in-ignored-dir
   echo 'ignored/' > .gitignore
   git add .gitignore && git commit -m "init"
   mkdir -p ignored/other
-  cp -Rv ../expendable-and-precious ignored/d
+  cp -R ../expendable-and-precious ignored/d
   rm -Rf ignored/d/*-by-filematch ignored/d/some-*
   mkdir -p other/ignored && >other/ignored/a
 )