Change default revision to absolute path

Change default revision from relative path "master" to absolute path
"refs/heads/master". "repo start <local-branch> <project>" uses this
value as-is to fill the branch merge line in .git/config files:

[branch "master"]
  remote = gfiber-internal
  merge = refs/heads/master

This change enables "repo start" to create proper local tracking branches.
The tracking status is visible with "git status":

gfiber-client/kernel/bruno$ git status
  On branch master
  Your branch is up-to-date with 'gfiber-internal/master'.

See b/13096078 for more details.

Change-Id: I3a3c6bdac6e98d9c4f523ae1ec2e32acbe3480d4
diff --git a/default.xml b/default.xml
index 4b3b059..858f914 100644
--- a/default.xml
+++ b/default.xml
@@ -1,10 +1,13 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <manifest>
   <remote
-    fetch="https://gfiber-internal.googlesource.com/"
     name="gfiber-internal"
+    fetch="https://gfiber-internal.googlesource.com/"
     review="https://gfiber-internal-review.googlesource.com/"/>
-  <default remote="gfiber-internal" revision="master" sync-j="12"/>
+  <default
+    remote="gfiber-internal"
+    revision="refs/heads/master"
+    sync-j="12"/>
   <manifest-server
     url="http://gfiber-autobot.mtv.corp.google.com:8000/manifest"/>