gitweb: '--cc' for merges in 'commitdiff' view

gitweb: '--cc' for merges in 'commitdiff' view

Allow choosing between '-c' (combined diff) and '--cc' (compact
combined) diff format in 'commitdiff' view for merge (multiparent)
commits.  Default is now '--cc'.

In the bottom part of navigation bar there is link allowing to change
diff format: "combined" for '-c' (when using '--cc') and "compact" for
'--cc' (when using '-c'), just on the right of "raw" link to
'commitdiff_plain" view.

About patchset part of diff --cc output: the difftree (whatchanged
table) has "patch" links to anchors to individual patches (on the same
page). The --cc option further compresses the patch output by
omitting some hunks; when this optimization makes all hunks disappear,
the patch is not shown (like in any other "empty diff" case). But the
fact that patch has been simplified out is not reflected in the raw
(difftree) part of diff output; the raw part is the same for '-c' and
'--cc' options. As correcting difftree is rather out of the question,
as it would require scanning patchset part before writing out
difftree, we add "Simple merge" empty diffs as a place to have anchor
to in place of those simplified out and removed patches.

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