+# The other head is given. Make sure it is valid.
+other=$(git-rev-parse --verify "$1^0") || usage
+
+# Make sure the branch to rebase is valid.
+head=$(git-rev-parse --verify "${2-HEAD}^0") || exit
+
+# If a hook exists, give it a chance to interrupt
+if test -x "$GIT_DIR/hooks/pre-rebase"
+then
+ "$GIT_DIR/hooks/pre-rebase" ${1+"$@"} || {
+ echo >&2 "The pre-rebase hook refused to rebase."
+ exit 1
+ }
+fi
+