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

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

When commit shown is a merge commit (has more than one parent),
display combined difftree output (result of git-diff-tree -c).
Earlier (since commit 549ab4a30703012ff3a12b5455d319216805a8db)
difftree output (against first parent) was not printed for merges.

Examples of non-trivial merges:
  5bac4a671907604b5fb4e24ff682d5b0e8431931 (includes rename)
  addafaf92eeb86033da91323d0d3ad7a496dae83 (five parents)
  95f97567c1887d77f3a46b42d8622c76414d964d (evil merge)

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