sync: Allow -j to have a default in manifest

This permits manifest authors to suggest a number of parallel
fetch operations against a remote server. For example, Gerrit
Code Review servers support queuing of requests and processes
them in first-in, first-out order. Running concurrent fetches
can utilize multiple CPUs on the Gerrit server, but will also
decrease overall operation latency by having the request put
into the queue ready to execute as soon as a CPU is free.

Change-Id: I3d3904acb6f63516bae4b071c510ad57a2afab18
Signed-off-by: Shawn O. Pearce <sop@google.com>

manifest_xml.py

@@ -29,6 +29,7 @@ class _Default(object):
 
   revisionExpr = None
   remote = None
+  sync_j = 1
 
 class _XmlRemote(object):
   def __init__(self,
@@ -133,6 +134,9 @@ class XmlManifest(object):
     if d.revisionExpr:
       have_default = True
       e.setAttribute('revision', d.revisionExpr)
+    if d.sync_j > 1:
+      have_default = True
+      e.setAttribute('sync-j', '%d' % d.sync_j)
     if have_default:
       root.appendChild(e)
       root.appendChild(doc.createTextNode(''))
@@ -401,6 +405,11 @@ class XmlManifest(object):
     d.revisionExpr = node.getAttribute('revision')
     if d.revisionExpr == '':
       d.revisionExpr = None
+    sync_j = node.getAttribute('sync-j')
+    if sync_j == '' or sync_j is None:
+      d.sync_j = 1
+    else:
+      d.sync_j = int(sync_j)
     return d
 
   def _ParseNotice(self, node):