Merge branch 'ab/i18n-scripts-basic'

* ab/i18n-scripts-basic: Makefile: add xgettext target for *.sh files git-sh-i18n.sh: add GIT_GETTEXT_POISON support git-sh-i18n.sh: add no-op gettext() and eval_gettext() wrappers git-sh-i18n--envsubst: our own envsubst(1) for eval_gettext()