contrib/rerere-train: use installed git-sh-setup

Instead of sourcing git-sh-setup from random place that is on the $PATH,
explicitly source $(git --exec-path)/git-sh-setup.  As I do not personally
have any libexec/git-core directory on my $PATH like many other people, the
script will fail without this update.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/rerere-train.sh

@@ -7,7 +7,7 @@ USAGE="$me rev-list-args"
 
 SUBDIRECTORY_OK=Yes
 OPTIONS_SPEC=
-. git-sh-setup
+. $(git --exec-path)/git-sh-setup
 require_work_tree
 cd_to_toplevel