branch_util: remove non-inclusive language from test and repo harness

Due to an initiative to remove non-inclusive language from code
git initiates repos with a main branch rather than a master. This
causes issues in the test and repo harness since it was built to
recognize master branches.

In this change all instances of master were replaced with main.
The unit tests were also changed to reflect the branch naming change.

Bug: b/171253379
Test: run_test.sh
Change-Id: Id327064149c2bfc5a6e1865b5e1ae67ee8b68d33
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2490606
Tested-by: Julio Hurtado <juahurta@google.com>
Reviewed-by: Mike Nichols <mikenichols@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: Julio Hurtado <juahurta@google.com>
6 files changed