project: initialize new manifests in temp dirs

If initializing the manifest fails for any reason, don't leave it in
a half complete state.  This can cause problems if/when the user tries
to reinit because different codepaths will be taken.  For example, if
we initialize manifests.git and don't finish probing the remote to see
what default branch it uses, we end up always using "master" even if
that isn't what the remote uses.

To avoid all of this, use .tmp dirs when initializing, and rename to
the final path only after we complete all the right steps.

We should roll this out to all projects we clone, but start with the
manifest project for now.

Bug: https://crbug.com/gerrit/13526
Bug: https://crbug.com/gerrit/15805
Change-Id: I0214338de69ee11e090285c6b0b211052804af06
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343539


Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Mike Frysinger <vapier@google.com>