android_merge_from_upstream: fix up naming

This replaces `master` with `main` where possible, per the linked bug.

It also switches to a more concise way of making sure we're never on the
local branch.

BUG=chromium:1099035
TEST=Ran the script in android-llvm

Change-Id: I0e656057acec857a2c91480cbd822a5bc6778563
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2265045
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed