gitweb: Fix unintended "--no-merges" for regular Atom feed

The print_feed_meta() subroutine generates links for feeds with and
without merges, in RSS and Atom formats.  However because %href_params
was not properly reset, it generated links with "--no-merges" for all
except the very first link.

Before:
<link rel="alternate" title="[..] - Atom feed" href="/?p=.git;a=atom;opt=--no-merges" type="application/atom+xml" />
<link rel="alternate" title="[..] - Atom feed (no merges)" href="/?p=.git;a=atom;opt=--no-merges" type="application/atom+xml" />

After:
<link rel="alternate" title="[..] - Atom feed" href="/?p=.git;a=atom" type="application/atom+xml" />
<link rel="alternate" title="[..] - Atom feed (no merges)" href="/?p=.git;a=atom;opt=--no-merges" type="application/atom+xml" />

Signed-off-by: Sebastian Pipping <sebastian@pipping.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -3886,6 +3886,7 @@ sub print_feed_meta {
 				'-type' => "application/$type+xml"
 			);
 
+			$href_params{'extra_options'} = undef;
 			$href_params{'action'} = $type;
 			$link_attr{'-href'} = href(%href_params);
 			print "<link ".