Install git-subtree from contrib

Install git-subtree from contrib

Build git-subtree in its contrib directory and install from there.
The main Makefile no longer discovers subcommands build in the main
build area so we cannot count on it to install git-subtree.  The user
should make && make install in contrib/subtree to install git-subtree.

Change the rule to install the git-subtree manpage.  The main
Documentation area doesn't directly support installing documentation
from other directories so the user will have to do that from within
contrib/subtree for now.

Signed-off-by: David A. Greene <greened@obbligato.org>