This is not a command the end user would want to run. Ever.
This documentation is meant for people who are studying the
plumbing scripts and/or are writing new ones.
git sh-i18n--envsubst is Git’s stripped-down copy of the GNU
envsubst(1) program that comes with the GNU gettext package. It’s
used internally by git-sh-i18n(1) to interpolate the variables
passed to the eval_gettext function.
No promises are made about the interface, or that this
program won’t disappear without warning in the next version
of Git. Don’t use it.