use git diff when generating release.h to check for dirty status

mkreleasehdr.sh

 #!/bin/sh
 GIT_SHA1=$((git show-ref --head --hash=8 2> /dev/null || echo 00000000) | head -n1)
-GIT_DIRTY=$(git status -s 2> /dev/null | wc -l)
+GIT_DIRTY=$(git diff 2> /dev/null | wc -l)
 test -f release.h || touch release.h
 (cat release.h | grep SHA1 | grep $GIT_SHA1) && \
 (cat release.h | grep DIRTY | grep $GIT_DIRTY) && exit 0 # Already uptodate