gitweb: make "No commits" in project list gray, not bold green

gitweb: make "No commits" in project list gray, not bold green

A missing return statement in git_get_last_activity made gitweb think
a project with no commits was in age class "age0", so the "No commits"
appeared in bold green, which was ridiculous.  I added the return so
those projects get "noage" and added a block to gitweb.css to format
the "No commits" text gray.

Signed-off-by: Matt McCutchen <redacted>
Signed-off-by: Junio C Hamano <redacted>