Set defaultbranch to python in .gitreview.

Avoid reviews being accidentally submitted on the wrong branch
by setting the default branch in the .gitreview file.

Change-Id: I2b7571920ec03bdadce3e352b53a0e632a509342
diff --git a/.gitreview b/.gitreview
index 90a22b1..fa97faa 100644
--- a/.gitreview
+++ b/.gitreview
@@ -2,3 +2,4 @@
 host=review.linaro.org
 port=29418
 project=toolchain/llvm/linaro-scripts
+defaultbranch=python