Update local downstream branches on renaming branch

When a branch tracks another local branch, downstream branches should
continue tracking the upstream branch even if it is renamed.  However,
renaming a branch does not automatically update the branch.<name>.merge
configuration option for its downstream branches to point to the new name
of the branch.  This causes the downstream branch to continue to point
to a `branch with the old name which may now no longer exist or may be a
completely different branch.

Made rename_branch() update the value of branch.<name>.merge to the new
name of the branch if it was earlier equal to the old name of the branch
and branch.<name>.remote was equal to ".".  Added a test case in t3200
to catch this bug.