Fixing the p4merge launch shell script

Hi,

I've noticed that the p4merge shell script could do with some improvement when it comes to merging. Because p4merge throws up an error when one of the files it's given to diff is "/dev/null", git needs to create a temporary empty file and pass that to p4merge when diffing a file that has been created/deleted (eg. create file, git add ., git diff --cached).

At the moment, the hack I'm using, which works, is to create a c:/blank.txt file which is an empty file, and modify "libexec/git-core/mergetools/p4merge" to start with this:

diff_cmd () {
	if [ ! -f "$LOCAL" ]
	then
		LOCAL="C:/blank.txt"
	fi
	if [ ! -f "$REMOTE" ]
	then
		REMOTE="C:/blank.txt"
	fi
	"$merge_tool_path" "$LOCAL" "$REMOTE"
}

Obviously, this isn't good enough because c:/blank.txt probably won't exist on the system. It would be nice for the temporary blank file to have the same extension as the one that's been added, so we could diff (say) c:/users/jez/Temp/abc123_newFile.foo c:/development/bar/newfile.foo. However, I can't see a way to do this purely from within the shell script (even `mktemp` doesn't exist on all systems so that isn't good enough). I believe git needs to create this temporary blank file itself, much like it creates a temporary file for the previous version of a file that's been modified when it's being diff'd. It then needs to expose the location of this temporary file as a variable; say $LOCALBLANK. So, we would be able to modify the shell script to this:

diff_cmd () {
	if [ ! -f "$LOCAL" ]
	then
		LOCAL=$LOCALBLANK
	fi
	if [ ! -f "$REMOTE" ]
	then
		REMOTE=$REMOTEBLANK
	fi
	"$merge_tool_path" "$LOCAL" "$REMOTE"
}

Thoughts?  Is there an easier way to do this?