gitweb: Unify the way long timestamp is displayed

format_timestamp_html loses its "-localtime => 1" option, and now
always print the local time (in author/comitter/tagger local
timezone), with "atnight" warning if needed.

This means that both 'summary' and 'log' views now display localtime.
In the case of 'log' view this can be thought as an improvement, as
now one can easily see which commits in a series are made "atnight"
and should be examined closer.

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