make: fix linter in git worktrees

When running `make lint` in a git worktree, the diff processor fails
with "no version control repository found" because the Docker container
only mounts the worktree directory, not the main git directory that the
worktree's .git file references.

This causes golangci-lint's `new-from-rev` filter to not work, resulting
in all ~32k existing lint issues being reported instead of only newly
introduced ones.

Fix by detecting when we're in a worktree (.git is a file, not a
directory) and mounting the main .git directory into the container so
revgrep can access the git history.

Makefile

@@ -75,20 +75,33 @@ endif
 # Paths inside container must match GOCACHE/GOMODCACHE in tools/Dockerfile.
 ifdef CI
 # CI mode: bind mount to host paths that GitHub Actions caches.
-DOCKER_TOOLS = docker run \
+DOCKER_TOOLS_BASE = docker run \
   --rm \
   -v $${HOME}/.cache/go-build:/tmp/build/.cache \
   -v $${HOME}/go/pkg/mod:/tmp/build/.modcache \
   -v $${HOME}/.cache/golangci-lint:/root/.cache/golangci-lint \
-  -v $$(pwd):/build lnd-tools
+	-v $$(pwd):/build
+DOCKER_TOOLS = $(DOCKER_TOOLS_BASE) lnd-tools
+DOCKER_TOOLS_LINT = $(DOCKER_TOOLS)
 else
 # Local mode: Docker named volumes for fast macOS/Windows performance.
-DOCKER_TOOLS = docker run \
+# Detect if we're in a git worktree. Use git rev-parse --git-common-dir to get
+# the path to the main git directory for the linter's diff processor to work
+# correctly with the new-from-rev setting.
+GIT_COMMON_DIR := $(shell \
+  common_dir="$$(git rev-parse --git-common-dir 2>/dev/null)"; \
+  if [ "$$common_dir" != ".git" ] && [ -n "$$common_dir" ]; then \
+    echo "$$common_dir"; \
+  fi)
+GIT_VOLUME := $(if $(GIT_COMMON_DIR),-v "$(GIT_COMMON_DIR):$(GIT_COMMON_DIR):ro",)
+DOCKER_TOOLS_BASE = docker run \
   --rm \
   -v lnd-go-build-cache:/tmp/build/.cache \
   -v lnd-go-mod-cache:/tmp/build/.modcache \
   -v lnd-go-lint-cache:/root/.cache/golangci-lint \
-  -v $$(pwd):/build lnd-tools
+  -v $$(pwd):/build
+DOCKER_TOOLS = $(DOCKER_TOOLS_BASE) lnd-tools
+DOCKER_TOOLS_LINT = $(DOCKER_TOOLS_BASE) $(GIT_VOLUME) lnd-tools
 endif
 
 GREEN := "\\033[0;32m"
@@ -358,7 +371,7 @@ check-go-version: check-go-version-dockerfile check-go-version-yaml
 #? lint-source: Run static code analysis
 lint-source: docker-tools	
 	@$(call print, "Linting source.")
-	$(DOCKER_TOOLS) custom-gcl run -v $(LINT_WORKERS)
+	$(DOCKER_TOOLS_LINT) custom-gcl run -v $(LINT_WORKERS)
 
 #? lint-config-check: Verify that the lint config is up to date
 #  We use the official linter here not our custom one because for checking the