Deprecate BitmapBuilder.add and introduce simpler addObject method

Deprecate BitmapBuilder.add and introduce simpler addObject method

The BitmapIndex.BitmapBuilder.add API is subtle:

	/**
	 * Adds the id and the existing bitmap for the id, if one
	 * exists, to the bitmap.
	 *
	 * @return true if the value was not contained or able to be
	 * loaded.
	 */
	boolean add(AnyObjectId objectId, int type);

Reading the name of the method does not make it obvious what it will
do.  Does it add the named object to the bitmap, or all objects
reachable from it?  It depends on whether the BitmapIndex owns an
existing bitmap for that object.  I did not notice this subtlety when
skimming the javadoc, either.  This resulted in enough confusion to
subtly break the bitmap building code (see change
I30844134bfde0cbabdfaab884c84b9809dd8bdb8 for details).

So discourage use of the add() API by deprecating it.

To replace it, provide a addObject() method that adds a single object.
This way, callers can decide whether to use addObject() or or() based
on the context.

For example,

	if (bitmap.add(c, OBJ_COMMIT)) {
		for (RevCommit p : c.getParents()) {
			rememberToAlsoHandle(p);
		}
	}

can be replaced with

	if (bitmap.contains(c)) {
		// already included
	} else if (index.getBitmap(c) != null) {
		bitmap.or(index.getBitmap(c));
	} else {
		bitmap.addObject(c, OBJ_COMMIT);
		for (RevCommit p : c.getParents()) {
			rememberToAlsoHandle(p);
		}
	}

which is more verbose but makes it clearer that the behavior
depends on the content of index.getBitmaps().

Change-Id: Ib745645f187e1b1483f8587e99501dfcb7b867a5