gitweb: Show combined diff for merge commits in 'commitdiff' view

gitweb: Show combined diff for merge commits in 'commitdiff' view

When 'commitdiff' action is requested without 'hp' (hash parent)
parameter, and commit given by 'h' (hash) parameter is merge commit,
show merge as combined diff.

Earlier for merge commits without 'hp' parameter diff to first parent
was shown.

Note that in compact combined (--cc) format 'uninteresting' hunks
omission mechanism can make that there is no patch corresponding to
line in raw format (difftree) output. That is why (at least for now)
we use --combined and not --cc format for showing commitdiff for merge
commits.

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