cvsserver: imitate git-update-ref when committing

git-update-ref writes into the lockfile, and renames it afterwards. Like
commit v1.3.0-rc3~22, it is not only cleaner, but also helps with shared
setups: every developer can have a different primary group; what matters
is that $GIT_DIR/refs/heads has to be writable by a group you are in.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-cvsserver.perl

@@ -1142,9 +1142,7 @@ sub req_ci
         exit;
     }
 
-    open FILE, ">", "$ENV{GIT_DIR}refs/heads/$state->{module}";
-    print FILE $commithash;
-    close FILE;
+    print LOCKFILE $commithash;
 
     $updater->update();
 
@@ -1171,7 +1169,9 @@ sub req_ci
     }
 
     close LOCKFILE;
-    unlink($lockfile);
+    my $reffile = "$ENV{GIT_DIR}refs/heads/$state->{module}";
+    unlink($reffile);
+    rename($lockfile, $reffile);
     chdir "/";
 
     print "ok\n";