Documentation: make AsciiDoc links always point to HTML files

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>
Merge branch 'ss/doclinks' into pu

* ss/doclinks:
  Documentation: make AsciiDoc links always point to HTML files
### notyet
Merge branch 'fc/at-head' into pu

* fc/at-head:
  DO NOT MERGE YET
  Add new @ shortcut for HEAD
  sha1-name: pass len argument to interpret_branch_name()