gitweb: Refactor common parts of 'log' and 'shortlog' views

gitweb: Refactor common parts of 'log' and 'shortlog' views

Put the common parts of git_log and git_shortlog into git_log_generic
subroutine: git_log and git_shortlog are now thin wrappers calling
git_log_generic with appropriate arguments.

The unification of code responsible for 'log' and 'shorlog' actions
lead to the following changes in gitweb output
 * 'tree' link in page_nav now uses $hash parameter, as was the case
   for 'shortlog' but not for 'log'
 * 'log' view now respect $hash_parent limiting, like 'shortlog' did
 * 'log' view doesn't have special case for empty list anymore, and it
   always uses page_header linking to summary view, like 'shortlog'
   did.

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