Support a merge with conflicting gitlink change

merge-recursive did not support merging trees that have conflicting
changes in submodules they contain, and died.  Support it exactly the
same way as how it handles conflicting symbolic link changes --- mark it
as a conflict, take the tentative result from the current side, and
letting the caller resolve the conflict, without dying in merge_file()
function.

Also reword the error message issued when merge_file() has to die
because it sees a tree entry of type it does not support yet.

[jc: fixed up initial draft by Finn Arne Gangstad]

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-merge-one-file.sh

@@ -80,6 +80,10 @@ case "${1:-.}${2:-.}${3:-.}" in
 		echo "ERROR: $4: Not merging symbolic link changes."
 		exit 1
 		;;
+	*,160000,*)
+		echo "ERROR: $4: Not merging conflicting submodule changes."
+		exit 1
+		;;
 	esac
 
 	src2=`git-unpack-file $3`

merge-recursive.c

@@ -1046,14 +1046,16 @@ static struct merge_file_info merge_file(struct diff_filespec *o,
 
 			free(result_buf.ptr);
 			result.clean = (merge_status == 0);
-		} else {
-			if (!(S_ISLNK(a->mode) || S_ISLNK(b->mode)))
-				die("cannot merge modes?");
-
+		} else if (S_ISGITLINK(a->mode)) {
+			result.clean = 0;
+			hashcpy(result.sha, a->sha1);
+		} else if (S_ISLNK(a->mode)) {
 			hashcpy(result.sha, a->sha1);
 
 			if (!sha_eq(a->sha1, b->sha1))
 				result.clean = 0;
+		} else {
+			die("unsupported object type in the tree");
 		}
 	}