Install git-stage in exec-path

Install git-stage in exec-path

Earlier the plan was to eventually eradicate git-foo executables from the
filesystem for all the built-in commands, but when we released 1.6.0 we
decided not to do so.  Instead, it has been promised that by prepending
the output from $(git --exec-path) to your $PATH, you can keep using the
dashed form of commands.

This also allows "git stage" to appear in the autogenerated command list,
which is used to offer man pages by "git help" command.

Signed-off-by: Junio C Hamano <gitster@pobox.com>