Fix repo prune output to sort by branch name

We didn't always sort the output.  Now we do.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/project.py b/project.py
index 4056860..4780316 100644
--- a/project.py
+++ b/project.py
@@ -810,7 +810,7 @@
 
     if cb and cb not in kill:
       kill.append(cb)
-      kill.sort()
+    kill.sort()
 
     kept = []
     for branch in kill: