gettext: avoid using gettext if the locale dir is not present

In cc5e1bf99247 (gettext: avoid initialization if the locale dir is not
present, 2018-04-21) Git was taught to avoid a costly gettext start-up
when there are not even any localized messages to work with.

But we still called `gettext()` and `ngettext()` functions.

Which caused a problem in Git for Windows when the libgettext that is
consumed from the MSYS2 project stopped using a runtime prefix in
https://github.com/msys2/MINGW-packages/pull/10461

Due to that change, we now use an unintialized gettext machinery that
might get auto-initialized _using an unintended locale directory_:
`C:\mingw64\share\locale`.

Let's record the fact when the gettext initialization was skipped, and
skip calling the gettext functions accordingly.

This addresses CVE-2023-25815.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
