init: use the remote default manifest branch

Instead of hardcoding "master" as our default, use the remote server's
default branch instead.  For most people, this should be the same as
"master" already.  For projects moving to "main", it means we'll use
the new name automatically rather than forcing people to use -b main.

For repositories that never set up a default HEAD, we should still use
the historical "master" default.

Bug: https://crbug.com/gerrit/13339
Change-Id: I4117c81a760c9495f98dbb1111a3e6c127f45eba
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/280799
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>