build dashless "bin-wrappers" directory similar to installed bindir

build dashless "bin-wrappers" directory similar to installed bindir

The new bin-wrappers directory contains wrapper scripts
for executables that will be installed into the standard
bindir.  It explicitly does not contain most dashed-commands.
The scripts automatically set environment variables to run out
of the source tree, not the installed directory.

This will allow running the test suite without dashed commands in
the PATH.  It also provides a simplified way to test run custom
built git executables without installing them first.

bin-wrappers also contains wrappers for some test suite support
executables, where the test suite will soon make use of them.

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