AsciiDoc's "link" is supposed to create hyperlinks for HTML output, so
prefer a "link" to point to an HTML file instead of a text file if an HTML
version of the file is being generated. For RelNotes, keep pointing to
text files as no equivalent HTML files are generated.
If appropriate, also update the link description to not contain the linked
file's extension.
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>