cvsimport: silence regex warning appearing in Perl 5.22.

Since Perl 5.22, "A literal '{' should now be escaped in a pattern".
Silence the recently added warning by using \{ instead.

Signed-off-by: Christian Neukirchen <chneukirchen@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-cvsimport.perl

@@ -921,7 +921,7 @@ sub commit {
 		# (See check_refname_component in refs.c.)
 		1 while $xtag =~ s/
 			(?: \.\.        # Tag cannot contain '..'.
-			|   \@{         # Tag cannot contain '@{'.
+			|   \@\{        # Tag cannot contain '@{'.
 			| ^ -           # Tag cannot begin with '-'.
 			|   \.lock $    # Tag cannot end with '.lock'.
 			| ^ \.          # Tag cannot begin...