Refactor part of GetUploadableBranches to lookup one specific branch

This way project.GetUploadableBranch(project.CurrentBranch) can tell
us how (if at all) to upload the currently checked out branch.

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

project.py

@@ -428,14 +428,22 @@ class Project(object):
       if branch in pubed and pubed[branch] == id:
         continue
 
-      branch = self.GetBranch(branch)
-      base = branch.LocalMerge
-      if branch.LocalMerge:
-        rb = ReviewableBranch(self, branch, base)
-        if rb.commits:
-          ready.append(rb)
+      rb = self.GetUploadableBranch(branch)
+      if rb:
+        ready.append(rb)
     return ready
 
+  def GetUploadableBranch(self, branch_name):
+    """Get a single uploadable branch, or None.
+    """
+    branch = self.GetBranch(branch_name)
+    base = branch.LocalMerge
+    if branch.LocalMerge:
+      rb = ReviewableBranch(self, branch, base)
+      if rb.commits:
+        return rb
+    return None
+
   def UploadForReview(self, branch=None):
     """Uploads the named branch for code review.
     """