'build' scripts before installing.

'build' scripts before installing.

Earlier we renamed git-foo.sh to git-foo while installing, which
was mostly done by inertia than anything else.  This however
made writing tests to use scripts harder.

This patch builds the scripts the same way as we build binaries
from their sources.  As a side effect, you can now specify
non-standard paths you have your Perl binary is in when running
the make.

Signed-off-by: Junio C Hamano <junkio@cox.net>