build: Ignore staged changes on up-to-date check

Have `make check-uptodate` ignore staged changes so you can run `make
ci` with all your changes staged to verify all is good before
committing. The `check-uptodate` target leaves files dirty in the
workspace so this change should not alter the original desired
behaviour.

`git status --porcelain` outputs a non-space character as the first
character on a line for staged changes, unless it is `?` which means
untracked.

Makefile

@@ -42,7 +42,7 @@ cover: test  ## Show test coverage in your browser
 
 check-uptodate: proto
 	go mod tidy
-	test -z "$$(git status --porcelain)" || { git diff; false; }
+	test -z "$$(git status --porcelain | grep '^[ ?]')" || { git diff; false; }
 
 CHECK_COVERAGE = awk -F '[ \t%]+' '/^total:/ {print; if ($$3 < $(COVERAGE)) exit 1}'
 FAIL_COVERAGE = { echo '$(COLOUR_RED)FAIL - Coverage below $(COVERAGE)%$(COLOUR_NORMAL)'; exit 1; }