i18n: add GETTEXT_POISON to simulate unfriendly translator

Add a new GETTEXT_POISON compile-time parameter to make _(msg) always
return gibberish. So now you can run

	make GETTEXT_POISON=YesPlease

to get a copy of git that functions correctly (one hopes) but produces
output that is in nobody's native language at all.

This is a debugging aid for people who are working on the i18n part of
the system, to make sure that they are not marking plumbing messages
that should never be translated with _().

As new strings get marked for translation, naturally a number of tests
will be broken in this mode. Tests that depend on output from
Porcelain will need to be marked with the new C_LOCALE_OUTPUT test
prerequisite. Newly failing tests that do not depend on output from
Porcelain would be bugs due to messages that should not have been
marked for translation.

Note that the string we're using ("# GETTEXT POISON #") intentionally
starts the pound sign. Some of Git's tests such as
t3404-rebase-interactive.sh rely on interactive editing with a fake
editor, and will needlessly break if the message doesn't start with
something the interactive editor considers a comment.

A future patch will fix fix the underlying cause of that issue by
adding "#" characters to the commit advice automatically.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -216,6 +216,9 @@ all::
 #
 # Define NO_REGEX if you have no or inferior regex support in your C library.
 #
+# Define GETTEXT_POISON if you are debugging the choice of strings marked
+# for translation.  This will turn all strings that use gettext into gibberish.
+#
 # Define JSMIN to point to JavaScript minifier that functions as
 # a filter to have gitweb.js minified.
 #
@@ -1370,6 +1373,9 @@ endif
 ifdef NO_SYMLINK_HEAD
 	BASIC_CFLAGS += -DNO_SYMLINK_HEAD
 endif
+ifdef GETTEXT_POISON
+	BASIC_CFLAGS += -DGETTEXT_POISON
+endif
 ifdef NO_STRCASESTR
 	COMPAT_CFLAGS += -DNO_STRCASESTR
 	COMPAT_OBJS += compat/strcasestr.o
@@ -2089,6 +2095,7 @@ endif
 ifdef GIT_TEST_CMP_USE_COPIED_CONTEXT
 	@echo GIT_TEST_CMP_USE_COPIED_CONTEXT=YesPlease >>$@
 endif
+	@echo GETTEXT_POISON=\''$(subst ','\'',$(subst ','\'',$(GETTEXT_POISON)))'\' >>$@
 
 ### Detect Tck/Tk interpreter path changes
 ifndef NO_TCLTK

gettext.h

@@ -15,9 +15,15 @@
 
 #define FORMAT_PRESERVING(n) __attribute__((format_arg(n)))
 
+#ifdef GETTEXT_POISON
+#define use_gettext_poison() 1
+#else
+#define use_gettext_poison() 0
+#endif
+
 static inline FORMAT_PRESERVING(1) const char *_(const char *msgid)
 {
-	return msgid;
+	return use_gettext_poison() ? "# GETTEXT POISON #" : msgid;
 }
 
 /* Mark msgid for translation but do not translate it. */

t/test-lib.sh

@@ -1079,6 +1079,9 @@ esac
 test -z "$NO_PERL" && test_set_prereq PERL
 test -z "$NO_PYTHON" && test_set_prereq PYTHON
 
+# Can we rely on git's output in the C locale?
+test -z "$GETTEXT_POISON" && test_set_prereq C_LOCALE_OUTPUT
+
 # test whether the filesystem supports symbolic links
 ln -s x y 2>/dev/null && test -h y 2>/dev/null && test_set_prereq SYMLINKS
 rm -f y