Fix doc versioning banner

Currently doc builds for `release_22.01` and `master` branches
incorrectly show the "This document is for an in-development version
of Galaxy." warning banner.

Partially backport commit 318a1d7056
and drop the unused `TARGET_BRANCH` variable from `Makefile` which
shadows the shell environment variable of the same name set
in `.github/workflows/docs.yaml` and used in
`doc/source/conf.versioning.py` .

Makefile

@@ -4,7 +4,6 @@ VENV?=.venv
 IN_VENV=if [ -f "$(VENV)/bin/activate" ]; then . "$(VENV)/bin/activate"; fi;
 RELEASE_CURR:=22.01
 RELEASE_UPSTREAM:=upstream
-TARGET_BRANCH=$(RELEASE_UPSTREAM)/dev
 CONFIG_MANAGE=$(IN_VENV) python lib/galaxy/config/config_manage.py
 PROJECT_URL?=https://github.com/galaxyproject/galaxy
 DOCS_DIR=doc
@@ -38,8 +37,8 @@ docs-develop: ## Fast doc generation and more warnings (for development)
 setup-venv:
 	if [ ! -f $(VENV)/bin/activate ]; then bash scripts/common_startup.sh --dev-wheels; fi
 
-diff-format:
-	$(IN_VENV) darker -r $(TARGET_BRANCH)
+diff-format:  ## Format Python code changes since last commit
+	$(IN_VENV) darker .
 
 list-dependency-updates: setup-venv
 	$(IN_VENV) pip list --outdated --format=columns