Use the local branch name as topic only if it is different from remote

The manpage already mentions:

"Alternatively, the local branch name will be used if different from
remote branch."

But actually local branch name was always used: simply don't specify a
topic if it would be the same as the branch name to avoid generating
unnecessary noise.

Change-Id: Idd7ac66d0df19a7abdae86f08174e1e37c29a19b