launcher: avoid crash when executing out of checkout

When developing repo itself, it helps to run repo directly out of it
and to run bisection tools.  The current _SetDefaultsTo logic fails
in that situation though as it wants a branch, but the source isn't
checked out to one.  Now that we support tracking commits via the
--repo-rev setting, fall back to using the current HEAD commit.

Change-Id: I37d79fd9f7bea87d212421ebed6c8267ec95145f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/260192
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>