GC: Trim more EWAHCompressedBitmaps to free unused memory

04b9f4436 fixed places where compressed bitmaps were holding on to their
full buffers, but missed this StoredBitmap.getBitmap() case where a
bitmap is resonstituted from an xor chain.

Change-Id: I7cf75d9e49c18a1a8a880a4df7e821502edc68a4
Signed-off-by: Terry Parker <tparker@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/BasePackBitmapIndex.java

@@ -104,6 +104,7 @@ abstract class BasePackBitmapIndex extends PackBitmapIndex {
 				r = xb.xorBitmap.bitmapContainer;
 				if (r instanceof EWAHCompressedBitmap) {
 					out = out.xor((EWAHCompressedBitmap) r);
+					out.trim();
 					bitmapContainer = out;
 					return out;
 				}