Update meta-project-migrate to pass "branch" to splitSubtree()

[kaelansmith]

It seems that splitSubtree() was set up to accept a "branch" parameter, with a default value set to "master"; and the meta-project-migrate script already seems to accept a final command-line argument for "branch" (which isn't documented btw), HOWEVER, the script doesn't actually pass the user-specified "branch" through to splitSubtree() -- so if your main branch isn't called "master" (like mine), the migrate script doesn't work. This PR adds a single-line fix, although untested.

[kaelansmith]

I now see that this previous PR also addressed this issue and more: #95 -- perhaps it needs to be revisited, or we can at least just merge my simple fix to handle the specific issue of customizing branch name?

I also just saw this past issue: mateodelnorte/meta#281 -- perhaps customizing the branch name via command-line argument doesn't fix all branch-related issues, and the concept of adding a defaultBranch within the .meta config is the better solution (which PR#95 added). If so, should we split up PR#95 so it only focuses on the branch issue and merge that?