git-sh-setup: be explicit where to dot-source git-sh-i18n from.

git-sh-setup: be explicit where to dot-source git-sh-i18n from.

d323c6b641 ("i18n: git-sh-setup.sh: mark strings for translation",
2016-06-17) started to dot-source git-sh-i18n shell script library,
assuming that $PATH is already adjusted for our scripts, namely,
$GIT_EXEC_PATH is at the beginning of $PATH.

Old contrib scripts like contrib/convert-grafts-to-replace-refs.sh
and contrib/rerere-train.sh and third-party scripts like guilt may
however be using this as ". $(git --exec-path)/git-sh-setup",
without satisfying that assumption.  Be more explicit by specifying
its path prefixed with "$(git --exec-path)/". to be safe.

While we’re here, move the sourcing of git-sh-i18n below the shell
portability fixes.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>