enhance Makefile for language translators

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

Makefile

@@ -2,6 +2,7 b' PREFIX=/usr/local'
 export PREFIX
 PYTHON=python
 PURE=
+PYTHON_FILES:=$(shell find mercurial hgext doc -name '*.py')
 
 help:
 	@echo 'Commonly used make targets:'
@@ -76,7 +77,9 b' tests:'
 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
@@ -86,10 +89,13 b' update-pot:'
         # 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