apache.conf.in: add mime_magic_module directive

apache.conf.in: add mime_magic_module directive

If mod_mime_magic is enabled, add a directive to turn it off.

When mod_mime_magic is enabled, it inspects the contents being
returned to provide a default Content-Type.  Unfortunately,
git-http-backend does not always provide a Content-Type and
when it does not mod_mime_magic adds the type of the script
rather than using text/plain.  This typically only happens
for errors but is clearly wrong.

Disable mod_mime_magic if enabled by setting its magic file
to /dev/null.  This also speeds up access to non-smart loose
objects and packs by avoiding the need to inspect them.