Bugfix: undefined htmldir in config.mak.autogen

Html documents will be installed to root dir (/) no matter what prefix
is set, if run these commands before `make` and `make install-html`:

    $ make configure
    $ ./configure --prefix=<PREFIX>

After the installation, all the html documents will copy to rootdir (/),
and:

    $ git --html-path
    <PREFIX>

    $ git help -w something
    fatal: '<PREFIX>': not a documentation directory.

This is because the variable "htmldir" points to a undefined variable
"$(docdir)" in file "config.mak.autogen", which is generated by running
`./configure`. By default $(docdir) generated by configure is supposed
be set this way:

        datarootdir='${prefix}/share'
        htmldir='${docdir}'
        docdir='${datarootdir}/doc/${PACKAGE_TARNAME}'

but since fc1c5415d69d (Honor configure's htmldir switch, 2013-02-02),
we only set and export htmldir without doing so for PACKAGE_TARNAME
(which is set to 'git' by the configure script).

Add the required two variables "PACKAGE_TARNAME" and "docdir" to file
"config.mak.in" will work this issue around.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>