Add presubmit check for tabs in html files.

Makefile

@@ -14,6 +14,7 @@
 
 VETTERS = "asmdecl,assign,atomic,bools,buildtag,cgocall,composites,copylocks,errorsas,httpresponse,loopclosure,lostcancel,nilfunc,printf,shift,stdmethods,structtag,tests,unmarshal,unreachable,unsafeptr,unusedresult"
 GOFMT_FILES = $(shell go list -f '{{.Dir}}' ./...)
+HTML_FILES = $(shell find . -name \*.html)
 
 fmtcheck:
 	@command -v goimports > /dev/null 2>&1 || go get golang.org/x/tools/cmd/goimports
@@ -30,6 +31,14 @@ fmtcheck:
 		fi
 .PHONY: fmtcheck
 
+tabcheck:
+	@CHANGES="$$(awk '/\t/ {print FILENAME,FNR}' $(HTML_FILES))"; \
+		if [ -n "$${CHANGES}" ]; then \
+			echo "$${CHANGES}\n\n"; \
+			exit 1; \
+		fi
+.PHONY: tabcheck
+
 spellcheck:
 	@command -v misspell > /dev/null 2>&1 || go get github.com/client9/misspell/cmd/misspell
 	@misspell -locale="US" -error -source="text" **/*

scripts/presubmit.sh

@@ -53,6 +53,13 @@ make spellcheck || {
 }
 
 
+echo "↹ Verify tabs"
+make tabcheck || {
+  echo "✋ Found tabs in html."
+  exit 1
+}
+
+
 echo "🔨 Building"
 go build ./...