Flush the pack header as soon as its ready

When the output stream is deeply buffered (e.g. 1 MiB or more in
an HTTP servlet on some containers) trying to kick out the header
earlier will prevent the client from stalling hard while the first
1 MiB is received and it can process the pack header.  Forcing a
flush here lets the client see the header and start its progress
monitor for "Receiving objects: (1/N)" so the user knows there
is still activity occurring, even though the buffering may cause
there to be some lag as the buffer fills up on the sending side.

Change-Id: I3edf39e8f703fe87a738dc236d426b194db85e3a
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackWriter.java

@@ -487,6 +487,7 @@ public class PackWriter {
 		int objCnt = getObjectsNumber();
 		writeMonitor.beginTask(JGitText.get().writingObjects, objCnt);
 		out.writeFileHeader(PACK_VERSION_GENERATED, objCnt);
+		out.flush();
 		writeObjects(writeMonitor, out);
 		writeChecksum(out);