Makefile: add "po-update" rule to update po/XX.po

Since there is no longer a "po/git.pot" file in tree, a l10n team leader
has to run several commands to update their "po/XX.po" file:

    $ make pot
    $ msgmerge --add-location --backup=off -U po/XX.po po/git.pot

To make this process easier, add a new rule so that l10n team leaders
can update their "po/XX.po" with one command. E.g.:

    $ make po-update PO_FILE=po/zh_CN.po

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -571,6 +571,7 @@ TCLTK_PATH = wish
 XGETTEXT = xgettext
 MSGCAT = msgcat
 MSGFMT = msgfmt
+MSGMERGE = msgmerge
 CURL_CONFIG = curl-config
 GCOV = gcov
 STRIP = strip
@@ -2715,6 +2716,7 @@ XGETTEXT_FLAGS_SH = $(XGETTEXT_FLAGS) --language=Shell \
 	--keyword=gettextln --keyword=eval_gettextln
 XGETTEXT_FLAGS_PERL = $(XGETTEXT_FLAGS) --language=Perl \
 	--keyword=__ --keyword=N__ --keyword="__n:1,2"
+MSGMERGE_FLAGS = --add-location --backup=off --update
 LOCALIZED_C = $(sort $(FOUND_C_SOURCES) $(FOUND_H_SOURCES) $(SCALAR_SOURCES) \
 	        $(GENERATED_H))
 LOCALIZED_SH = $(sort $(SCRIPT_SH) git-sh-setup.sh)
@@ -2792,6 +2794,22 @@ po/git.pot: .build/pot/git.header $(LOCALIZED_ALL_GEN_PO)
 .PHONY: pot
 pot: po/git.pot
 
+define check_po_file_envvar
+	$(if $(PO_FILE), \
+		$(if $(filter po/%.po,$(PO_FILE)), , \
+			$(error PO_FILE should match pattern: "po/%.po")), \
+		$(error PO_FILE is not defined))
+endef
+
+.PHONY: po-update
+po-update: po/git.pot
+	$(check_po_file_envvar)
+	@if test ! -e $(PO_FILE); then \
+		echo >&2 "error: $(PO_FILE) does not exist"; \
+		exit 1; \
+	fi
+	$(QUIET_MSGMERGE)$(MSGMERGE) $(MSGMERGE_FLAGS) $(PO_FILE) po/git.pot
+
 .PHONY: check-pot
 check-pot: $(LOCALIZED_ALL_GEN_PO)
 

shared.mak

@@ -63,6 +63,7 @@ ifndef V
 	QUIET_LNCP     = @echo '   ' LN/CP $@;
 	QUIET_XGETTEXT = @echo '   ' XGETTEXT $@;
 	QUIET_MSGFMT   = @echo '   ' MSGFMT $@;
+	QUIET_MSGMERGE = @echo '   ' MSGMERGE $@;
 	QUIET_GCOV     = @echo '   ' GCOV $@;
 	QUIET_SP       = @echo '   ' SP $<;
 	QUIET_HDR      = @echo '   ' HDR $(<:hcc=h);