Improve makefile

Makefile

@@ -13,10 +13,16 @@
 BRANCH := $(shell git branch --show-current)
 
 # Branches representing docs for older Python versions, which current
-# translations should be merged into.
-# Check Python Docs versions still being built and published in:
-# https://github.com/python/docsbuild-scripts/blob/master/build_docs.py
-MERGEBRANCHES := 3.8 3.7 3.6 2.7
+# translations should be merged into. Here some details:
+# - STABLEBRANCH and OLDSTABLEBRANCHES differ in that STABLEBRANCH
+# still gets its translated strings overwritten.
+# - Not-yet-translated strings will still be merged on both variables
+# - Leave STABLEBRANCH empty if Python stable branch is same as BRANCH.
+# - Never use same version as in BRANCH
+# - Check Python Docs versions still being built and published in:
+# https://github.com/python/docsbuild-scripts/blob/master/build_docs.py
+STABLEBRANCH := 3.8
+OLDSTABLEBRANCHES := 3.7 3.6 2.7
 
 # Name of language team; should be python-docs-LANG, where LANG is the
 # IETF language tag for your language; see Language Tag section in PEP 545
@@ -53,7 +59,8 @@ help:
 @echo " pot Create/Update POT files from source files"
 @echo " serve Serve a built documentation on http://localhost:8000"
 @echo " spell Check spelling, storing output in $(POSPELL_TMP_DIR)"
-@echo " merge Merge $(BRANCH) branch's .po files into: $(MERGEBRANCHES)"
+@echo " merge Merge $(BRANCH) branch's .po files into the following"
+@echo " older branches: $(STABLEBRANCH) $(OLDSTABLEBRANCHES)"
 @echo ""
 
 
@@ -151,6 +158,11 @@ pot: setup
 # the translation files copy which could have new/updated files.
 .PHONY: setup
 setup: venv
+@if [ -z $(BRANCH) ]; then \
+echo "BRANCH is empty, should have git-branch. Unable to continue."; \
+exit 1; \
+fi
+
 @if ! [ -d $(CPYTHON_PATH) ]; then \
 echo "CPython repo not found; cloning ..."; \
 git clone --depth 1 --no-single-branch $(UPSTREAM) $(CPYTHON_PATH); \
@@ -221,25 +233,32 @@ $(POSPELL_TMP_DIR)/typos.txt:
 
 
 # merge: merge translations from BRANCH (Python version currently aim of
-# translation) into each branch listed by MERGEBRANCHES (branches
-# of older Python versions) so that older versions of the Python
-# docs make at least some use the latest translations, if possible.
+# translation) into each branch listed by STABLEBRANCH and
+# OLDSTABLEBRANCHES (branches of older Python versions) so that older
+# versions of the Python Docs make at least some use the latest
+# translations, if possible. OLDSTABLEBRANCHES has '--no-overwrite'
+# flag so it does not overwrite translated strings, preserving history.
 # After merging, git-push merged files (if any) to the target branch.
 .PHONY: merge
-merge: venv $(MERGEBRANCHES)
-
-$(MERGEBRANCHES):
-@echo "Merging translations from $(BRANCH) branch into $@ ..."
-@$(VENV)/bin/pomerge --from-files *.po **/*.po
-@git checkout $@
-@$(VENV)/bin/pomerge --no-overwrite --to-files *.po **/*.po
-@$(VENV)/bin/powrap --modified *.po **/*.po
-@if git status -s | egrep '\.po'; then \
-git add *.po **/*.po; \
-git commit -m "pomerge from $(BRANCH) branch into $@"; \
-git push; \
+merge: venv $(STABLEBRANCH) $(OLDSTABLEBRANCHES)
+
+$(OLDSTABLEBRANCHES): OVERWRITEFLAG = --no-overwrite
+$(STABLEBRANCH) $(OLDSTABLEBRANCHES):
+@if [ $@ == $(BRANCH) ]; then \
+echo "Ignoring attempt to pomerge '$(BRANCH)' into itself"; \
+else \
+echo "Merging translations from $(BRANCH) branch into $@ ..."; \
+$(VENV)/bin/pomerge --from-files *.po **/*.po; \
+git checkout $@; \
+$(VENV)/bin/pomerge $(OVERWRITEFLAG) --to-files *.po **/*.po; \
+$(VENV)/bin/powrap --modified *.po **/*.po; \
+if git status -s | egrep '\.po'; then \
+git add *.po **/*.po; \
+git commit -m "pomerge from $(BRANCH) branch into $@"; \
+git push; \
+fi; \
+git checkout $(BRANCH); \
 fi
-@git checkout $(BRANCH)
 
 
 # clean: remove all .mo files and the venv directory that may exist and could