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
diff --git a/agl_services_build/newrepo.sh b/agl_services_build/newrepo.sh
index a5f512a..524b541 100755
--- a/agl_services_build/newrepo.sh
+++ b/agl_services_build/newrepo.sh
@@ -26,6 +26,7 @@
 git init > /dev/null
 git add default.xml
 git commit -am "Manifest" > /dev/null
+git_branch_name=$(git branch --show-current)
 cd "../repo"
-yes | repo init -u ../manifest -q
+yes | repo init -u ../manifest -b ${git_branch_name} -q
 repo sync -c -q -j2