merge-one-file: compute empty blob object ID

This script hard-codes the object ID of the empty blob.  To avoid any
problems when changing hashes, compute this value by calling git
hash-object.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-merge-one-file.sh b/git-merge-one-file.sh
index 9879c59..f6d9852 100755
--- a/git-merge-one-file.sh
+++ b/git-merge-one-file.sh
@@ -120,7 +120,7 @@
 	case "$1" in
 	'')
 		echo "Added $4 in both, but differently."
-		orig=$(git unpack-file e69de29bb2d1d6434b8b29ae775ad8c2e48c5391)
+		orig=$(git unpack-file $(git hash-object /dev/null))
 		;;
 	*)
 		echo "Auto-merging $4"