Ensure git status is tracked at the correct time

The previous version would verify git status at time of `make verify` execution, but before all its dependencies executed.
If one of the dependencies resulted in a dirty git, the check would still pass.

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

Makefile

@@ -225,13 +225,12 @@ ifneq ($(shell grep -o 'LIBGIT2_TAG ?= \w.*' Makefile | cut -d ' ' -f 3), $(shel
 	exit 1; \
 	}
 endif
-ifneq (, $(shell git status --porcelain --untracked-files=no))
-	@{ \
+
+	@if [ ! "$$(git status --porcelain --untracked-files=no)" = "" ]; then \
 		echo "working directory is dirty:"; \
 		git --no-pager diff; \
 		exit 1; \
-	}
-endif
+	fi
 
 # go-install-tool will 'go install' any package $2 and install it to $1.
 define go-install-tool