gitweb: Fix thinko in git_tags and git_heads

gitweb: Fix thinko in git_tags and git_heads

git_get_refs_list always return reference to list (and reference to
hash which we ignore), so $taglist (in git_tags) and $headlist (in
git_heads) are always defined, but @$taglist / @$headlist might be
empty. Replaced incorrect "if (defined @$taglist)" with
"if (@$taglist)" in git_tags and respectively in git_heads.

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