Makefile: extract Q_() source strings as ngettext()

The Q_() wrapper added by 0c9ea33 (i18n: add stub Q_() wrapper for
ngettext, 2011-03-09) needs to be noticed by xgettext.

Add an appropriate --keyword option to the Makefile, so that "make pot"
would notice the strings in the plural form marked with the wrapper.

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

@@ -2051,7 +2051,8 @@ XGETTEXT_FLAGS = \
 	--add-comments \
 	--msgid-bugs-address="Git Mailing List <git@vger.kernel.org>" \
 	--from-code=UTF-8
-XGETTEXT_FLAGS_C = $(XGETTEXT_FLAGS) --keyword=_ --keyword=N_ --language=C
+XGETTEXT_FLAGS_C = $(XGETTEXT_FLAGS) --language=C \
+	--keyword=_ --keyword=N_ --keyword="Q_:1,2"
 LOCALIZED_C := $(C_OBJ:o=c)
 
 po/git.pot: $(LOCALIZED_C)