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
1 file changed
tree: 6a66a926d8d939eba2f24a84ba918a37a6cd9eae
  1. default.xml