Add `init.templatedir` configuration variable.

Add `init.templatedir` configuration variable.

Rather than having to pass --template to git init and clone for a custom
setup, `init.templatedir` may be set in '~/.gitconfig'.  The environment
variable GIT_TEMPLATE_DIR can already be used for this but this is nicer.

System administrators may prefer using this variable in the system-wide
config file to point at a locally modified copy (e.g. /etc/gittemplate)
rather than editing vanilla template files in '/usr/share'.

Signed-off-by: Steven Drake <sdrake@xnet.co.nz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>