gitweb: refer to picon/gravatar images over the same scheme

With the current code, the images from picon and gravatar are
requested over http://, and browsers give mixed contents warning
when gitweb is served over https://.

Just drop the scheme: part from the URL, so that these external
sites are accessed over https:// in such a case.

Signed-off-by: Andrej E Baranov <redacted>
Signed-off-by: Junio C Hamano <redacted>