gitfaq: avoid validation error with older asciidoc

gitfaq: avoid validation error with older asciidoc

When building with asciidoc-8.4.5 (as found on CentOS/Red Hat 6), the
period in the "[[files-in-.gitignore-are-tracked]]" anchor is not
properly parsed as a section:

  WARNING: gitfaq.txt: line 245: missing [[files-in-.gitignore-are-tracked]] section

The resulting XML file fails to validate with xmlto:

    xmlto: /git/Documentation/gitfaq.xml does not validate (status 3)
    xmlto: Fix document syntax or use --skip-validation option
     /git/Documentation/gitfaq.xml:3: element refentry: validity error :
     Element refentry content does not follow the DTD, expecting
     (beginpage? , indexterm* , refentryinfo? , refmeta? , (remark | link
     | olink | ulink)* , refnamediv+ , refsynopsisdiv? , (refsect1+ |
     refsection+)), got (refmeta refnamediv refsynopsisdiv refsect1
     refsect1 refsect1 refsect1 variablelist refsect1 refsect1 )
    Document /git/Documentation/gitfaq.xml does not validate

Let's avoid breaking users of platforms which ship an old version of
asciidoc, since the cost to do so is quite low.

Reported-by: Son Luong Ngoc <sluongng@gmail.com>
Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>