DfsMidxWriter: Do not accept midx as input to build midx

We can use a midx to build a new midx, but it breaks the bitmaps. When
we calculate the "midx offsets" to order objects in the bitmap, we
assume each pack is a single pack (one packId).

Throw an exception if the input for the midx is another midx, until we
calculate properly the midx offsets.

Link: https://review.gerrithub.io/id/I325443dc3281003b29b4d8a1598b83316a6a6964

Author: ifradeo

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsMidxWriter.java

@@ -77,6 +77,13 @@ public static DfsPackDescription writeMidx(ProgressMonitor pm,
 		PackIndexMerger.Builder dataBuilder = PackIndexMerger.builder();
 		try (DfsReader ctx = objdb.newReader()) {
 			for (DfsPackFile pack : packs) {
+				if (pack instanceof DfsPackFileMidx) {
+					// asObjectsToPack assumes one packid per pack while
+					// calculating midx offsets. We need to fix that before
+					// accepting midx to build new midx.
+					throw new IOException(
+							"MIDX not supported to build new midx"); //$NON-NLS-1$
+				}
 				dataBuilder.addPack(pack.getPackDescription().getPackName(),
 						pack.getPackIndex(ctx));
 			}