enhance Makefile for language translators

To update a po-file just do 'make i18n/xx.po'. No other po-file will
be touched.

Makefile

@@ -1,95 +1,101
 PREFIX=/usr/local
 export PREFIX
 PYTHON=python
 PURE=
+PYTHON_FILES:=$(shell find mercurial hgext doc -name '*.py')
 
 help:
 	@echo 'Commonly used make targets:'
 	@echo '  all          - build program and documentation'
 	@echo '  install      - install program and man pages to PREFIX ($(PREFIX))'
 	@echo '  install-home - install with setup.py install --home=HOME ($(HOME))'
 	@echo '  local        - build for inplace usage'
 	@echo '  tests        - run all tests in the automatic test suite'
 	@echo '  test-foo     - run only specified tests (e.g. test-merge1)'
 	@echo '  dist         - run all tests and create a source tarball in dist/'
 	@echo '  clean        - remove files created by other targets'
 	@echo '                 (except installed files or dist source tarball)'
 	@echo '  update-pot   - update i18n/hg.pot'
 	@echo
 	@echo 'Example for a system-wide installation under /usr/local:'
 	@echo '  make all && su -c "make install" && hg version'
 	@echo
 	@echo 'Example for a local installation (usable in this directory):'
 	@echo '  make local && ./hg version'
 
 all: build doc
 
 local:
 	$(PYTHON) setup.py $(PURE) build_py -c -d . build_ext -i build_mo
 	$(PYTHON) hg version
 
 build:
 	$(PYTHON) setup.py $(PURE) build
 
 doc:
 	$(MAKE) -C doc
 
 clean:
 	-$(PYTHON) setup.py clean --all # ignore errors of this command
 	find . -name '*.py[cdo]' -exec rm -f '{}' ';'
 	rm -f MANIFEST mercurial/__version__.py mercurial/*.so tests/*.err
 	rm -rf locale
 	$(MAKE) -C doc clean
 
 install: install-bin install-doc
 
 install-bin: build
 	$(PYTHON) setup.py $(PURE) install --prefix="$(PREFIX)" --force
 
 install-doc: doc
 	cd doc && $(MAKE) $(MFLAGS) install
 
 install-home: install-home-bin install-home-doc
 
 install-home-bin: build
 	$(PYTHON) setup.py $(PURE) install --home="$(HOME)" --force
 
 install-home-doc: doc
 	cd doc && $(MAKE) $(MFLAGS) PREFIX="$(HOME)" install
 
 MANIFEST-doc:
 	$(MAKE) -C doc MANIFEST
 
 MANIFEST: MANIFEST-doc
 	hg manifest > MANIFEST
 	echo mercurial/__version__.py >> MANIFEST
 	cat doc/MANIFEST >> MANIFEST
 
 dist:	tests dist-notests
 
 dist-notests:	doc MANIFEST
 	TAR_OPTIONS="--owner=root --group=root --mode=u+w,go-w,a+rX-s" $(PYTHON) setup.py -q sdist
 
 tests:
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS)
 
 test-%:
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS) $@
 
-update-pot:
+update-pot: i18n/hg.pot
+
+i18n/hg.pot: $(PYTHON_FILES)
 	mkdir -p i18n
 	pygettext -d hg -p i18n --docstrings \
 	  mercurial/commands.py hgext/*.py hgext/*/__init__.py
         # All strings marked for translation in Mercurial contain
         # ASCII characters only. But some files contain string
         # literals like this '\037\213'. xgettext thinks it has to
         # parse these them even though they are not marked for
         # translation. Extracting with an explicit encoding of
         # ISO-8859-1 will make xgettext "parse" and ignore them.
-	find mercurial hgext doc -name '*.py' | xargs \
+	echo $^ | xargs \
 	  xgettext --from-code ISO-8859-1 --join --sort-by-file \
 	  -d hg -p i18n -o hg.pot
 
+%.po: i18n/hg.pot
+	msgmerge --no-location --update $@ $^
+
 .PHONY: help all local build doc clean install install-bin install-doc \
 	install-home install-home-bin install-home-doc dist dist-notests tests \
 	update-pot