Update shell scripts to compute empty tree object ID

Several of our shell scripts hard-code the object ID of the empty tree.
To avoid any problems when changing hashes, compute this value on
startup of the script.  For performance, store the value in a variable
and reuse it throughout the life of the script.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -11,6 +11,8 @@
 # The following functions will also be available in the commit filter:
 
 functions=$(cat << \EOF
+EMPTY_TREE=$(git hash-object -t tree /dev/null)
+
 warn () {
 	echo "$*" >&2
 }
@@ -46,7 +48,7 @@ git_commit_non_empty_tree()
 {
 	if test $# = 3 && test "$1" = $(git rev-parse "$3^{tree}"); then
 		map "$3"
-	elif test $# = 1 && test "$1" = 4b825dc642cb6eb9a060e54bf8d69288fbee4904; then
+	elif test $# = 1 && test "$1" = $EMPTY_TREE; then
 		:
 	else
 		git commit-tree "$@"

git-rebase--interactive.sh

@@ -81,6 +81,8 @@ rewritten_pending="$state_dir"/rewritten-pending
 # and leaves CR at the end instead.
 cr=$(printf "\015")
 
+empty_tree=$(git hash-object -t tree /dev/null)
+
 strategy_args=${strategy:+--strategy=$strategy}
 test -n "$strategy_opts" &&
 eval '
@@ -238,7 +240,7 @@ is_empty_commit() {
 		die "$(eval_gettext "\$sha1: not a commit that can be picked")"
 	}
 	ptree=$(git rev-parse -q --verify "$1"^^{tree} 2>/dev/null) ||
-		ptree=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+		ptree=$empty_tree
 	test "$tree" = "$ptree"
 }
 

templates/hooks--pre-commit.sample

@@ -12,7 +12,7 @@ then
 	against=HEAD
 else
 	# Initial commit: diff against an empty tree object
-	against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+	against=$(git hash-object -t tree /dev/null)
 fi
 
 # If you want to allow non-ASCII filenames set this variable to true.