gitweb: make search help link less ugly

The search help link was a superscript question mark right next to
a drop-down menu, which looks misaligned and is a cramped and
awkward click target. Remove the superscript tags and add some
spacing to fix these nits. Add a title attribute to provide an
explanatory mouseover.

Signed-off-by: Tony Finch <redacted>
Signed-off-by: Junio C Hamano <redacted>