add: don't write objects with --dry-run

add: don't write objects with --dry-run

When the option --dry-run/-n is given, "git add" doesn't change the
index, but still writes out new object files.  Only hash the latter
without writing instead to make the run as dry as possible.

Use this opportunity to also make the hash_flags variable unsigned,
to match the index_path() parameter it is used as.

Reported-by: git.mexon@spamgourmet.com
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>