Merge branch 'ow/no-dryrun-in-add-i'

"git add -i --dry-run" does not dry-run, which was surprising.  The
combination of options has taught to error out.

* ow/no-dryrun-in-add-i:
  add: die if both --dry-run and --interactive are given

diff --cc builtin/add.c
Simple merge

diff --cc t/t3700-add.sh
Simple merge