luci.mk: fix compatibility with GNU Make 4.2

GNU Make version 4.2 chokes on `#` characters embedded in variable
substitutions, leading to the following error:

    ../../luci.mk:114: *** unterminated call to function 'if': missing ')'.  Stop.

Avoid this problem by replacing variable substitutions with Git branch
format arguments.

Ref: https://github.com/openwrt/luci/pull/5976#issuecomment-1289935106
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

luci.mk

@@ -114,9 +114,8 @@ PKG_SRC_VERSION?=$(if $(DUMP),x,$(strip $(call findrev,1)))
 PKG_GITBRANCH?=$(if $(DUMP),x,$(strip $(shell \
 	variant="LuCI"; \
 	if git log -1 >/dev/null 2>/dev/null; then \
-		branch=$$(git branch --remote --no-abbrev --contains 2>/dev/null | tail -n1); \
-		branch=$${branch:-$$(git branch --no-abbrev --contains 2>/dev/null)}; \
-		branch=$${branch#? }; branch=$${branch#*/}; \
+		branch=$$(git branch --format='%(refname:strip=3)' --remote --no-abbrev --contains 2>/dev/null | tail -n1); \
+		branch=$${branch:-$$(git branch --format='%(refname:strip=2)' --no-abbrev --contains 2>/dev/null | tail -n1)}; \
 		if [ "$$branch" != "master" ]; then \
 			variant="LuCI $${branch:-unknown} branch"; \
 		else \