dfs: handle short copies

`copy` is documented as possibly returning a smaller number of bytes
than requested. In practice, this can occur if a block is cached and the
reader never pulls in the file to check its size.

Bug: 565874
Change-Id: I1e53b3d2f4ab09334178934dc0ef74ea99045cd3
Signed-off-by: wh <wh9692@protonmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -607,8 +607,15 @@ public final class DfsPackFile extends BlockBasedFile {
 
 	private void readFully(long position, byte[] dstbuf, int dstoff, int cnt,
 			DfsReader ctx) throws IOException {
-		if (ctx.copy(this, position, dstbuf, dstoff, cnt) != cnt)
-			throw new EOFException();
+		while (cnt > 0) {
+			int copied = ctx.copy(this, position, dstbuf, dstoff, cnt);
+			if (copied == 0) {
+				throw new EOFException();
+			}
+			position += copied;
+			dstoff += copied;
+			cnt -= copied;
+		}
 	}
 
 	ObjectLoader load(DfsReader ctx, long pos)