Don't upload when dest branch is not merge branch

Example:
- `repo init -b master` / sync a project
- In one project: `git checkout -b work origin/branch-thats-not-master`
- make some changes, `git commit`
- `repo upload .`
- Upload will now be skipped with a warning instead of being uploaded to
  master

Change-Id: I990b36217b75fe3c8b4d776e7fefa1c7d9ab7282
1 file changed