sync: pass --bare option when doing git clone of superproject.

Changed "git pull" to "git fetch" as we are using --bare option. Used the
following command to fetch:
  git fetch origin +refs/heads/*:refs/heads/* --prune

Pass --branch argument to Superproject's UpdateProjectsRevisionId function.

Returned False/None when directories don't exist instead of raise
GitError exception from _Fetch and _LsTree functions. The caller of Fetch
does Clone if Fetch fails.

Tested the code with the following commands.

$ ./run_tests -v

Tested the init and sync code by copying all the repo changes into my Android
AOSP checkout and running repo sync with --use-superproject option.

Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707


Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I3e441ecdfc87c735f46eff0eb98efa63cc2eb22a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296222


Reviewed-by: Mike Frysinger <vapier@google.com>