IndexPack: Remove blob-streaming size threshold

Always use streaming (for SHA-checksum & collision detection)
when indexing whole blobs, regardless of their size.

Positives:
* benefits of bugfix #312868 will apply to all runtimes, without
  additional conf for mem-constrained JVMs (5MB huge for some)
* no byte array allocation
  (re-uses readBuffer instead of allocating new full-size array)
* mildly better overall performance
  (given the usual blob-does-not-need-collision-checking case)
* removes unnecessary code

Negative:
* doubles the disk IO for a blob comparision
  (comparitively rare occurance)

I perf-tested a range of threshold sizes against a random selection
of packfiles I found on my harddrive, the results are here:

https://spreadsheets.google.com/ccc?key=tLCQElyyd2RKN9QevfvgwGQ&hl=en_GB#gid=1

My interpretation of the results is that the streaming size threshold
isn't beneficial (actually seems to be very slightly detrimental) -so
we should just get rid of it. This tallies with some of the comments
Shawn & I had for the default value of streamFileThreshold in the
review for I862afd4c:

http://egit.eclipse.org/r/#patch,sidebyside,2040,2,org.eclipse.jgit/src/org/eclipse/jgit/transport/IndexPack.java

The perf-test code is here: https://gist.github.com/735402
It's a bit scruffy but basically does 10 runs (in randomised order)
for each threshold size on various packfiles, waiting a second
between each pack-indexing to allow GC to catch up. I know it's not
perfect - proper perf testing is hard to do :-)

org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/IndexPackTest.java

@@ -166,7 +166,6 @@ public class IndexPackTest extends RepositoryTestCase {
 
 		final byte[] raw = pack.toByteArray();
 		IndexPack ip = IndexPack.create(db, new ByteArrayInputStream(raw));
-		ip.setStreamFileThreshold(1);
 		ip.index(NullProgressMonitor.INSTANCE);
 		ip.renameAndOpenPack();
 	}

org.eclipse.jgit/src/org/eclipse/jgit/transport/IndexPack.java

@@ -145,8 +145,6 @@ public class IndexPack {
 
 	private final Repository repo;
 
-	private int streamFileThreshold;
-
 	/**
 	 * Object database used for loading existing objects
 	 */
@@ -244,7 +242,6 @@ public class IndexPack {
 	public IndexPack(final Repository db, final InputStream src,
 			final File dstBase) throws IOException {
 		repo = db;
-		streamFileThreshold = 5 * (1 << 20); // A reasonable default for now.
 		objectDatabase = db.getObjectDatabase().newCachedDatabase();
 		in = src;
 		inflater = new InflaterStream();
@@ -268,10 +265,6 @@ public class IndexPack {
 		}
 	}
 
-	void setStreamFileThreshold(int sz) {
-		streamFileThreshold = sz;
-	}
-
 	/**
 	 * Set the pack index file format version this instance will create.
 	 *
@@ -860,7 +853,7 @@ public class IndexPack {
 		objectDigest.update((byte) 0);
 
 		boolean checkContentLater = false;
-		if (type == Constants.OBJ_BLOB && sz >= streamFileThreshold) {
+		if (type == Constants.OBJ_BLOB) {
 			InputStream inf = inflate(Source.INPUT, sz);
 			long cnt = 0;
 			while (cnt < sz) {