cvsimport: shell-quote variable used in backticks

We run `git rev-parse` though the shell, and quote its
argument only with single-quotes. This prevents most
metacharacters from being a problem, but misses the obvious
case when $name itself has single-quotes in it. We can fix
this by applying the usual shell-quoting formula.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-cvsimport.perl

@@ -642,6 +642,7 @@ sub is_sha1 {
 
 sub get_headref ($) {
 	my $name = shift;
+	$name =~ s/'/'\\''/;
 	my $r = `git rev-parse --verify '$name' 2>/dev/null`;
 	return undef unless $? == 0;
 	chomp $r;