Enable color for diffs in consistency checks

Drone CI can display terminal colors, so force-enable it to make diffs
more readable on it.

Makefile

@@ -273,7 +273,7 @@ fmt:
 
 .PHONY: fmt-check
 fmt-check: fmt
- @diff=$$(git diff $(GO_SOURCES) templates $(WEB_DIRS)); \
+ @diff=$$(git diff --color=always $(GO_SOURCES) templates $(WEB_DIRS)); \
  if [ -n "$$diff" ]; then \
  echo "Please run 'make fmt' and commit the result:"; \
  echo "$${diff}"; \
@@ -309,7 +309,7 @@ $(SWAGGER_SPEC): $(GO_SOURCES_NO_BINDATA)
 
 .PHONY: swagger-check
 swagger-check: generate-swagger
- @diff=$$(git diff '$(SWAGGER_SPEC)'); \
+ @diff=$$(git diff --color=always '$(SWAGGER_SPEC)'); \
  if [ -n "$$diff" ]; then \
  echo "Please run 'make generate-swagger' and commit the result:"; \
  echo "$${diff}"; \
@@ -414,7 +414,7 @@ vendor: go.mod go.sum
 
 .PHONY: tidy-check
 tidy-check: tidy
- @diff=$$(git diff go.mod go.sum $(GO_LICENSE_FILE)); \
+ @diff=$$(git diff --color=always go.mod go.sum $(GO_LICENSE_FILE)); \
  if [ -n "$$diff" ]; then \
  echo "Please run 'make tidy' and commit the result:"; \
  echo "$${diff}"; \
@@ -885,7 +885,7 @@ svg: node-check | node_modules
 .PHONY: svg-check
 svg-check: svg
  @git add $(SVG_DEST_DIR)
- @diff=$$(git diff --cached $(SVG_DEST_DIR)); \
+ @diff=$$(git diff --color=always --cached $(SVG_DEST_DIR)); \
  if [ -n "$$diff" ]; then \
  echo "Please run 'make svg' and 'git add $(SVG_DEST_DIR)' and commit the result:"; \
  echo "$${diff}"; \
@@ -895,7 +895,7 @@ svg-check: svg
 .PHONY: lockfile-check
 lockfile-check:
  npm install --package-lock-only
- @diff=$$(git diff package-lock.json); \
+ @diff=$$(git diff --color=always package-lock.json); \
  if [ -n "$$diff" ]; then \
  echo "package-lock.json is inconsistent with package.json"; \
  echo "Please run 'npm install --package-lock-only' and commit the result:"; \