autogen.sh now configures git only on request

autogen.sh now configures git only on request

* autogen.sh (do_autoconf, do_git): New vars.
Support new arguments --help, all, autoconf, git.
By default, just do autoconf-related configuration, not git.
Prefer 'echo' to 'cat <<EOF ...', as this tends to avoid temp files.
If GNU cp is available, use it to backup .git/config before
changing it.  When configuring git, chatter about what is being
done, and configure git to check hashes.  Avoid some duplicate
file name specification when creating git hooks.

* GNUmakefile (ALL_IF_GIT): New macro.
(configure): Use it.
* INSTALL.REPO: Suggest './autogen.sh all'.