Use readFully() instead of read()

Fixes the "Method ignores results of InputStream.read()" warning.

This is the only place where read() was used instead of readFully()
and the return value was not checked. So it was either an oversight
or should be documented. This change assumes it was an oversight.

Change-Id: I859404a7d80449c538a552427787f3e57d7c92b4

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackFile.java

@@ -599,7 +599,7 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 					JGitText.get().packObjectCountMismatch, packCnt, idx.getObjectCount(), getPackFile()));
 
 		fd.seek(length - 20);
-		fd.read(buf, 0, 20);
+		fd.readFully(buf, 0, 20);
 		if (!Arrays.equals(buf, packChecksum))
 			throw new PackMismatchException(MessageFormat.format(
 					JGitText.get().packObjectCountMismatch