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

In cc5e1bf992 (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>

gettext.c

@@ -109,6 +109,8 @@ static void init_gettext_charset(const char *domain)
 		setlocale(LC_CTYPE, "C");
 }
 
+int git_gettext_enabled = 0;
+
 void git_setup_gettext(void)
 {
 	const char *podir = getenv(GIT_TEXT_DOMAIN_DIR_ENVIRONMENT);
@@ -130,6 +132,8 @@ void git_setup_gettext(void)
 	init_gettext_charset("git");
 	textdomain("git");
 
+	git_gettext_enabled = 1;
+
 	free(p);
 }