Merge branch 'et/add-chmod-x' into maint

"git update-index --add --chmod=+x file" may be usable as an escape
hatch, but not a friendly thing to force for people who do need to
use it regularly.  "git add --chmod=+x file" can be used instead.

* et/add-chmod-x:
add: add --chmod=+x / --chmod=-x options