patchcvsimport: always pass user data to "system" as a list

> This avoids invoking the shell. Not only is it faster, but
>  it prevents the possibility of interpreting our arguments in
>  the shell.
>
>  Signed-off-by: Jeff King <peff@peff.net>
>  ---
>  Nigel reported a problem with a CVS branch that had '(' in the name. No
>  report yet on whether this fixes it, but it should be done regardless.
>
>  I checked over git-cvsimport.perl, and this was the only instance I
>  found.
>
>   git-cvsimport.perl |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
>
>  diff --git a/git-cvsimport.perl b/git-cvsimport.perl
>  index 95c5eec..bdac5d5 100755
>  --- a/git-cvsimport.perl
>  +++ b/git-cvsimport.perl
>  @@ -772,7 +772,7 @@ sub commit {
>         waitpid($pid,0);
>         die "Error running git-commit-tree: $?\n" if $?;
>
>  -       system("git-update-ref $remote/$branch $cid") == 0
>  +       system('git-update-ref', "$remote/$branch", $cid) == 0
>                 or die "Cannot write branch $branch for update: $!\n";
>
>         if ($tag) {
>  --
>  1.5.5.1.178.gc1eb9.dirty
>
>