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

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

The executable bit will not be detected (and therefore will not be
set) for paths in a repository with `core.filemode` set to false,
though the users may still wish to add files as executable for
compatibility with other users who _do_ have `core.filemode`
functionality.  For example, Windows users adding shell scripts may
wish to add them as executable for compatibility with users on
non-Windows.

Although this can be done with a plumbing command
(`git update-index --add --chmod=+x foo`), teaching the `git-add`
command allows users to set a file executable with a command that
they're already familiar with.

Signed-off-by: Edward Thomson <ethomson@edwardthomson.com>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>