gitweb: Provide links to commitdiff to each parent in 'commitdiff' view

gitweb: Provide links to commitdiff to each parent in 'commitdiff' view

Since commit-fb1dde4a we show combined diff for merges in 'commitdiff'
view, and since commit-208ecb2e also in 'commit' view. Sometimes
though one would want to see diff to one of merge commit parents. It
is easy in 'commit' view: in the commit header part there are "diff"
links for each of parent header. This commit adds such links also for
'commitdiff' view.

Add to difftree / whatchanged table row with "1", "2", ... links to
'commitdiff' view for diff with n-th parent for merge commits, as a
table header.  This is visible only in 'comitdiff' view, and only for
a merge commit (comit with more than one parent).

To save space links are shown as "n", where "n" is number of a parent,
and not as for example shortened (to 7 characters) sha1 of a parent
commit.  To make it easier to discover what links is for, each link
has 'title' attribute explaining the link.

Note that one would need to remember that difftree table in 'commit'
view has one less column (it doesn't have "patch" link column), if one
would want to add such table header also in 'commit' view.

Example output:
                          1       2       3
  Makefile      patch | diff1 | diff2 | diff3 | blob | history
  cache.h       patch | diff1 | diff2 | diff3 | blob | history

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>