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 fc1c541 (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>

config.mak.in

@@ -9,6 +9,7 @@ CC_LD_DYNPATH = @CC_LD_DYNPATH@
 AR = @AR@
 TAR = @TAR@
 DIFF = @DIFF@
+PACKAGE_TARNAME = @PACKAGE_TARNAME@
 #INSTALL = @INSTALL@		# needs install-sh or install.sh in sources
 TCLTK_PATH = @TCLTK_PATH@
 
@@ -19,6 +20,7 @@ gitexecdir = @libexecdir@/git-core
 datarootdir = @datarootdir@
 template_dir = @datadir@/git-core/templates
 sysconfdir = @sysconfdir@
+docdir = @docdir@
 
 mandir = @mandir@
 htmldir = @htmldir@