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 <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>