INSTALL: document a simpler way to run uninstalled builds

INSTALL: document a simpler way to run uninstalled builds

The new scripts automatically saved in the bin-wrappers directory allow
you to run a build when you have neither installed git nor tweaked
environment variables.  Mention this in INSTALL, along with the slight
performance issue of doing so.

This can be especially handy for manually testing network-invoked git
(from ssh, web servers, or similar), but it is also handy with a plain
command prompt.

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>