gitweb: Refactor 'log' action generation, adding git_log_body()

Put the main part of 'log' view generation into git_log_body,
similarly how it is done for 'shortlog' and 'history' views (and
also for 'tags' and 'heads' views).

This is preparation for extracting common code between 'log',
'shortlog' and 'history' actions.

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