Topic name should be determined only for git push

When setting up git-review, it tries to acquire a topic name although
that is of no use to setup.  A side effect is that git-review can not be
setup whenever the git remote is a fresh repository since it get no
branch. Solve this by moving it after the setup routines.

Inform the user why the particular topic name has been selected.

Change-Id: Iaa942b8dc77b0c3f720085358c38e8295138025a
Check that the owner of a change has name defined

Sometimes Gerrit might not return owner name
due to some internal misconfiguration or database
problem. Since we are using owner name here to
generate the name of a local git branch, instead
of failing here we should try to just go ahead
with 'unknown'.

More information about this error:

https://bugzilla.wikimedia.org/show_bug.cgi?id=40061#c5
https://gerrit.wikimedia.org/r/#/c/37771/

Change-Id: Icaa9baa453e3fd7d54adf9138a9435e4271cee0a