Makefile: skip precheck if DESTDIR set

If DESTDIR is set assume an install is in progress and
that the actual final git binary may not be available
since the installation is to an alternate destination
and skip running the precheck.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
help: make tg help tg work

Make `tg help tg` show the README since that has the entire
overview and summary of each command.

Also arrange to install the README file as tg-tg.txt in the
same location as the other help files.