Allow inclusive branch names for manifest repository

git is moving away from `master` for an initial branch name into a more
proper and inclusive `main` nomenclature. In order to allow tools at
different stages in this transition to interoperate properly, add
support for detecting the default branch name for the manifest project.

Bug: None
Test: ran newrepo.sh, observed branch name `main`
Change-Id: I6672b74e49195f4a53ddfa0f38b1a55fced7b84e
1 file changed