Documentation: Strip texinfo anchors to avoid duplicates

This keeps texinfo 5.x happy. See https://bugs.gentoo.org/464210.

Signed-off-by: Martin von Gagern <Martin.vGagern@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'mg/texinfo-5' into pu

* mg/texinfo-5:
  Documentation: Strip texinfo anchors to avoid duplicates