[PATCH 39/41] Update shell scripts to compute empty tree object ID

[brian m. carlson]

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 <sand...@crustytoothpaste.net>
---
 git-filter-branch.sh               | 4 +++-
 git-rebase--interactive.sh         | 4 +++-
 templates/hooks--pre-commit.sample | 2 +-
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/git-filter-branch.sh b/git-filter-branch.sh
index 64f21547c1..ccceaf19a7 100755
--- a/git-filter-branch.sh
+++ b/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" = $EMPTY_TREE; then
                :
        else
                git commit-tree "$@"
diff --git a/git-rebase--interactive.sh b/git-rebase--interactive.sh
index 50323fc273..cc873d630d 100644
--- a/git-rebase--interactive.sh
+++ b/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"
 }
 
diff --git a/templates/hooks--pre-commit.sample 
b/templates/hooks--pre-commit.sample
index 68d62d5446..6a75641638 100755
--- a/templates/hooks--pre-commit.sample
+++ b/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.