Fix "already merging" safety check in merge-to-branch.sh

tools/merge-to-branch.sh

 #!/bin/bash
 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
@@ skipping 70 matching lines @@
         exit 1
         ;;
   esac
 done
 let OPTION_COUNT=$OPTIND-1
 shift $OPTION_COUNT
 
 ########## Regular workflow
 
 # If there is a merge in progress, abort.
-[[ -e "$ALREADY_MERGING_SENTINEL_FILE" ]] && [[ -z "$START_STEP" ]] \
+[[ -e "$ALREADY_MERGING_SENTINEL_FILE" ]] && [[ $START_STEP -eq 0 ]] \
    && die "A merge is already in progress"
 touch "$ALREADY_MERGING_SENTINEL_FILE"
 
 initial_environment_checks
 
 if [ $START_STEP -le $CURRENT_STEP ] ; then
   echo ">>> Step $CURRENT_STEP: Preparation"
   MERGE_TO_BRANCH=$1
   [[ -n "$MERGE_TO_BRANCH" ]] || die "Please specify a branch to merge to"
   shift
@@ skipping 132 matching lines @@
     https://v8.googlecode.com/svn/$TO_URL \
     https://v8.googlecode.com/svn/tags/$NEWMAJOR.$NEWMINOR.$NEWBUILD.$NEWPATCH \
     -m "Tagging version $NEWMAJOR.$NEWMINOR.$NEWBUILD.$NEWPATCH"
 fi
 
 let CURRENT_STEP+=1
 if [ $START_STEP -le $CURRENT_STEP ] ; then
   echo ">>> Step $CURRENT_STEP: Cleanup."
   common_cleanup
 fi