BUILD: Fix the gitstamp dirty flag not correctly updating

Yeah, if we're calling "update-index" instead of "git update-index",
no wonder it's fickle...

gitstamp/Makefile.am

@@ -5,7 +5,7 @@ LIBS =
 .PHONY: gitstamp
 gitstamp:
 	@-$(eval $@_REVLINE := $(shell git describe --long --match desc/\* | cut -d '/' -f 2- | sed -e 's/\(.*\)-\([^-]*\)-\([^-]*\)/\1+\2.\3/'))
-	@-$(eval $@_REVDIRT := $(shell update-index --refresh --unmerged 1>/dev/null 2>&1; git diff-index --quiet HEAD || echo ".dirty"))
+	@-$(eval $@_REVDIRT := $(shell git update-index --refresh --unmerged 1>/dev/null 2>&1; git diff-index --quiet HEAD || echo ".dirty"))
 	@-$(eval $@_REVDESC := $(shell if test x$($@_REVLINE) != "x"; then echo $($@_REVLINE)$($@_REVDIRT); fi ))
 	@[ -f $@ ] || touch $@
 	$(AM_V_GEN)echo $($@_REVDESC) | cmp -s $@ - || echo $($@_REVDESC) > $@