gitweb: Fix file links in "grep" search

gitweb: Fix file links in "grep" search

There were two bugs in generating file links (links to "blob" view),
one hidden by the other.  The correct way of generating file link is

	href(action=>"blob", hash_base=>$co{'id'},
	     file_name=>$file);

It was $co{'hash'} (this key does not exist, and therefore this is
undef), and 'hash' instead of 'hash_base'.

To have this fix applied in single place, this commit also reduces
code duplication by saving file link (which is used for line links) in
$file_href.

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