i18n: add--interactive: mark plural strings

Mark plural strings for translation.  Unfold each action case in one
entire sentence.

Pass new keyword for xgettext to extract.

Update test to include new subroutine __n() for plural strings handling.

Update documentation to include a description of the new __n()
subroutine.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index df4f86b..12b56ce 100644
--- a/Makefile
+++ b/Makefile
@@ -2109,7 +2109,8 @@
 	--keyword=_ --keyword=N_ --keyword="Q_:1,2"
 XGETTEXT_FLAGS_SH = $(XGETTEXT_FLAGS) --language=Shell \
 	--keyword=gettextln --keyword=eval_gettextln
-XGETTEXT_FLAGS_PERL = $(XGETTEXT_FLAGS) --keyword=__ --language=Perl
+XGETTEXT_FLAGS_PERL = $(XGETTEXT_FLAGS) --language=Perl \
+	--keyword=__ --keyword="__n:1,2"
 LOCALIZED_C = $(C_OBJ:o=c) $(LIB_H) $(GENERATED_H)
 LOCALIZED_SH = $(SCRIPT_SH)
 LOCALIZED_SH += git-parse-remote.sh