gitweb: Simplify 'opt' parameter validation, add "no merges" feeds

gitweb: Simplify 'opt' parameter validation, add "no merges" feeds

Simplify and make more readable validation of 'opt' (extra options)
parameter, using exists($hash{key}) instead of grepping keys of a hash
for value.

Move 'opt' parameter to be the last (for now) in the URL.

Make use of '--no-merges' extra option ('opt') by adding "no merges"
RSS and Atom feeds to the HTML header.  Note that alternate format
links in the RSS and Atom views do not use '--no-merges' option yet!

Adds tests for the 'opt' parameter to t9500-gitweb-standalone-no-errors.sh

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>