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);
 }
 

gettext.h

@@ -31,9 +31,11 @@
 int use_gettext_poison(void);
 
 #ifndef NO_GETTEXT
+extern int git_gettext_enabled;
 void git_setup_gettext(void);
 int gettext_width(const char *s);
 #else
+#define git_gettext_enabled (0)
 static inline void git_setup_gettext(void)
 {
 	use_gettext_poison(); /* getenv() reentrancy paranoia */
@@ -48,7 +50,8 @@ static inline FORMAT_PRESERVING(1) const char *_(const char *msgid)
 {
 	if (!*msgid)
 		return "";
-	return use_gettext_poison() ? "# GETTEXT POISON #" : gettext(msgid);
+	return use_gettext_poison() ? "# GETTEXT POISON #" :
+		!git_gettext_enabled ? msgid : gettext(msgid);
 }
 
 static inline FORMAT_PRESERVING(1) FORMAT_PRESERVING(2)
@@ -56,6 +59,8 @@ const char *Q_(const char *msgid, const char *plu, unsigned long n)
 {
 	if (use_gettext_poison())
 		return "# GETTEXT POISON #";
+	if (!git_gettext_enabled)
+		return n == 1 ? msgid : plu;
 	return ngettext(msgid, plu, n);
 }