diff-tree -c: show a merge commit a bit more sensibly.

A new option '-c' to diff-tree changes the way a merge commit is
displayed when generating a patch output.  It shows a "combined
diff" (hence the option letter 'c'), which looks like this:

    $ git-diff-tree --pretty -c -p fec9ebf | head -n 18
    diff-tree fec9ebf... (from parents)
    Merge: 0620db3... 8a263ae...
    Author: Junio C Hamano <junkio@cox.net>
    Date:   Sun Jan 15 22:25:35 2006 -0800

	Merge fixes up to GIT 1.1.3

    diff --combined describe.c
    @@@ +98,7 @@@
	    return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
       }

    -  static void describe(char *arg)
     - static void describe(struct commit *cmit, int last_one)
    ++ static void describe(char *arg, int last_one)
       {
     +      unsigned char sha1[20];
     +      struct commit *cmit;

There are a few things to note about this feature:

 - The '-c' option implies '-p'.  It also implies '-m' halfway
   in the sense that "interesting" merges are shown, but not all
   merges.

 - When a blob matches one of the parents, we do not show a diff
   for that path at all.  For a merge commit, this option shows
   paths with real file-level merge (aka "interesting things").

 - As a concequence of the above, an "uninteresting" merge is
   not shown at all.  You can use '-m' in addition to '-c' to
   show the commit log for such a merge, but there will be no
   combined diff output.

 - Unlike "gitk", the output is monochrome.

A '-' character in the nth column means the line is from the nth
parent and does not appear in the merge result (i.e. removed
from that parent's version).

A '+' character in the nth column means the line appears in the
merge result, and the nth parent does not have that line
(i.e. added by the merge itself or inherited from another
parent).

The above example output shows that the function signature was
changed from either parents (hence two "-" lines and a "++"
line), and "unsigned char sha1[20]", prefixed by a " +", was
inherited from the first parent.

The code as sent to the list was buggy in few corner cases,
which I have fixed since then.

It does not bother to keep track of and show the line numbers
from parent commits, which it probably should.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -182,7 +182,7 @@ LIB_H = \
 
 DIFF_OBJS = \
 	diff.o diffcore-break.o diffcore-order.o diffcore-pathspec.o \
-	diffcore-pickaxe.o diffcore-rename.o tree-diff.o
+	diffcore-pickaxe.o diffcore-rename.o tree-diff.o combine-diff.o
 
 LIB_OBJS = \
 	blob.o commit.o connect.o count-delta.o csum-file.o \