Context:
In https://codereview.chromium.org/781523002/ I added the ability for target_ref to be set to any branch name. Eg: For Skia a ref of 'refs/remotes/origin/chrome/m42' would be correctly set to 'refs/heads/chrome/m42'.

But in https://codereview.chromium.org/822503005/ this was changed to treat any branch that did not start with 'refs/remotes/branch-heads' or 'refs/remotes/origin/refs' to be 'refs/remotes/origin/master'.
This makes it very chromium specific, there are plenty of other projects that use depot_tools that does not work like this.
For Skia 'refs/remotes/origin/chrome/m42' would now change to 'refs/remotes/origin/master' which is wrong.

The default behavior should be to preserve the original ref not to override it to master.

I handled 'lkgr' and 'lkcr' as special cases in this CL because many developers track it and would like it to land in master by default.

BUG=chromium:463109
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/967453004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294303 0039d316-1c4b-4281-b951-d872f2087c98